Phylum Rotifera

Colonial rotifers from a lake in Germany

Since I didn't have much to say about the last phylum, I decided to move on to the next phylum on my list: Rotifera. This is actually one of my favorites of the invertebrate phyla.

Rotifera comes from the Greek for "wheel bearer," and reflects one of the common names: wheel animals. This rather unusual-sounding common name comes from the ciliated corona that surrounds the mouth and is used to capture food. Due to cilia action, the corona appear to rotate like wheels when viewed under a microscope. There are approximately 1850 species known, with the vast majority of those species being found in freshwater. These animals are actually a common sight in pond water samples, especially those with a fair amount of debris. Samples of lake and pond average between forty and five hundred rotifers per liter. Most rotifers are free living, with parasitism being unusual in this phylum. Though some species are colonial, most are not. They aren't long-lived animals, with five weeks being an unusually long lifespan and one or two weeks being more typical.

The mastax is seen in the middle left

The corona is not the only interesting-looking moving structure found in rotifers. When viewed under a microscope, there is something that looks rather a lot like a beating heart. However, this is actually a unique muscular pharynx known as a mastax containing structures known as trophi. Trophi are used for grinding food, sucking food in, or grabbing prey. Interestingly enough, the trophi are so varied in shape between species that they play a very important part in species identification.

Rotifers were one part of the former Phylum Aschelminthes which has since been broken down into several smaller groups. One characteristics that almost all of the aschelminths share is a developmental trait known as eutely. Eutely is when cells stop dividing at a certain point in development and any further growth in the size of the animal happens because the cells that are present increase in size.

Looping movement in some rotifers

Movement in free-living species often involves swiming using the cilia in the corona. Rotifers have a foot with up to four toes. Glands opening in the toes secrete a substance that certain species will use to attach to surfaces temporarily. Feeding will often occur while the individual is attached, with coronal cilia being used to sweep food into the waiting mouth. Other species are constantly swimming and never attach to a surface.

The cementing substance that is secreted by the toes can also be used in the form of movement seen at left, where a combination of muscular contractions and attachment at the toes will draw the animal across a surface. This method is often called looping due to the body's resemblance to a U-shaped loop. This form of movement does not require swimming via the corona.

A member of Class Bdelloidea

Rotifer reproduction is one of the major characteristics to break the phylum into classes. Class Seisonidea includes only parasitic species  and all species reproduce sexually. Class Bdelloidea has only free-living species and all of those species reproduce asexually, with all known individuals being female. This class is also capable of forming into a cyst capable of withstanding long-term extreme conditions. Class Monogononta is the largest of the three and all of the species included have a rather interesting life cycle. Usually, reproduction is asexual via of the creation of amictic eggs that contain only the genes of the mother. This reproduction is very quick, with populations capable of doubling in less than a day. When conditions do not favor this, some of the eggs being produced are a bit different. They hatch into females that produce only mictic eggs that are like the eggs we produce: having only half of the normal amount of genetic information. If not fertilized, the egg will hatch into a small male. Males cannot feed and their only purpose in life is to fertilize mictic eggs. The fertilized eggs then go into a resting stage which can handle the harsh conditions that triggered sexual reproduction to occur. Overall, the vast majority of rotifers that you will find are females due to the characteristics of these three classes.

Source is Biology of the Invertebrates. Images are from Wikimedia Commons and are under Creative Commons licenses or copyright free: one, two, three, four.

Phylum Mesozoa

Today's animal phylum is a rather obscure one, so obscure that I wasn't able to find a properly licensed image to use. If it helps, here is an image of a member of Class Rhombozoa. 

Mesozoa comes from the Greek for "middle animal" and is a phylum that contains creatures having traits of both protists and flatworms. Though some include this on the list of animal phyla, others do not. This is mainly due to very fuzzy relationships to other animal groups. Studies have shown relationships to both the flat- and roundworms, along with some mesozoans being closer to segmented worms, molluscs, and other invertebrates. Despite similarities, these differences have lead some to split Mesozoa into several phyla based on closest relation. Others maintain their status as a single phylum.

Mesozoans are parasites and show many of the characteristics that are seen in other parasitic creatures. The body form of these animals is very simple, lacking a number of basic systems. They have no nervous system or digestive tract. This simplified body form is likely derived from more complex ancestors, with extra parts being lost when they had become parasitic. Their method of reproduction is quite unusual, with development occurring inside of other cells. No other animal group has shown this sort of method.

Phylum Mesozoa includes two classes: Class Orthonectida and Class Rhombozoa. Orthonectida members can parasitize a variety of invertebrate, marine hosts, while the Rhombozoa members are only found in the kidney-like nephridia of cephalopods.

Source is Biology of the Invertebrates.

Phylum Platyhelminthes

Many marine Turbelarian flatworms are quite beautiful

This group consists of what are called the flatworms. Indeed, the phylum name is from the Greek for flatworm. The group is rather varied and includes a number of parasites. Though many can be quite nasty parasites, there are also a number of species that are quite harmless and some that are even rather beautiful. There really isn't a single characteristic that can be used to define the phylum, except possibly the flat body. All flatworms are triploblastic acoelomates that are bilaterlally symmetrical. That is, they have three tissue layers but no body cavity and can only be evenly divided into mirror-image halves in one place. This is the first animal phyla that I have discussed that is triploblastic. It is also the first protostome species. This is a division among the animals based on very early development.

The phylum is broken down into a number of different classes, all of which are quite different.

Class Turbellaria
This class is mostly made up of free-living species, though a small number are parasitic or commensal (species that live off of another but do not cause the other species any harm). Though the vast majority of turbellarians live in water, there are a few species that are able to live on land. My favorite are these terrestrial land planarians.

Pork tapeworm proglottid, full of eggs

Class Cestoda
This class includes the tapeworms. Tapeworms can be quite nasty parasites, living in the gut of mostly vertebrate species. Nutrition is obtained via the outer body covering. They have a "head" or scolex that is little more than an attachment point, covered in hooks and suckers. Following that is a number of segments known as proglottids, whose main purpose is reproduction. It is estimates that up to one hundred thirty-five million people are currently living with tapeworm infections. Pork tapeworms can be especially nasty as worms can form cysts in the brain if a human becomes infected.

Class Monogenea
A relatively under-studied class, most species contained within are parasites of fish gills. Previously placed both in with the cestodes and the trematodes (which will be discussed next), molecular studies have placed them in their own separate class.

Class Trematoda

This fluke causes schistosomiasis

The best known members of this class are commonly known as flukes. One example, the blood fluke, causes schistosomiasis, which has an estimated two hundred million sufferers and kills approximately eight hundred thousand people each year. Trematodes often have quite complex life cycles, requiring up to three hosts and multiple changes in body form for the full cycle. Snails are an extremely common part of these life cycles, usually serving as an intermediate host before the parasite passes on to another host.  Common names of fluke species usually imply where the fluke lives, such as the blood flukes and liver flukes that are seen in humans. The group I have been discussing are known as Digeneans due to the two intermediate hosts seen in their life cycles. Trematodes also include a small group known as Aspidogastreans. These eighty species are parasites on molluscs and share traits with both monogeneans and digeneans.

Overall, the greatest concern surrounding Platyhelminthes revolves around the parasitic species, especially those which infect humans. Both cestodes and trematodes can be very serious and some are difficult to treat. Schistosomiasis is especially concerning due to its high infection rate. Research is being done to control the spread of the disease. The infections that are seen are usually restricted to underdeveloped countries, with few if any cases popping up in countries like the United States.

Source is Biology of the Invertebrates. Images are from Wikimedia Commons and are under Creative Commons licenses or are copyright free: one, two, three.

Phylum Ctenophora

One of the many deep-sea species found in Phylum Ctenophora. This species is bioluminescent.

It's been a while since I took a look at one of the animal phyla. I've spoken about this particular phylum in some detail before, so some of you may have already read parts of this post.

A larvae with a colored body

The name Ctenophora is derived from the Greek for "comb bearing," and this phylum includes approximately one hundred and fifty species. Though they superficially resemble jellyfish and other Cnidarians, the two groups are quite different. For one, Ctenophores lack the stinging cells that the Cnidarians have that cause the painful sensation when their tentacles brush against you. Instead of using those stinging cells to capture food, Ctenophores used structures called colloblasts, which secrete a sticky, adhesive substance. They are actually quite successful predators, to the point where some are troublesome invasives. These animals can also be distinguished from jellyfish and their relatives thanks to their complete digestive tract. While Cnidarians have only a blind-ended sack, comb jellies have a distinct mouth for ingestion of food and anal pores for elimination of waste.

Ctenophores are commonly referred to as comb jellyfish thanks to another of their characteristic features. Every Ctenophore has rows of cilia known as ctenes or combs organized into eight bands along the body called costae or comb rows. This is their primary method of movement and most species are only weak swimmers, unable to swim against a strong current. This makes them planktonic, despite the large size of some species. In combination with the combs, a sense organ opposite of their mouth is used to detect their orientation and controls how quickly the different costae beat.

A bioluminescent species

These creatures can be quite colorful, though many are transparent. In addition to body pigment, nearly every of species is bioluminescent. This is common in organisms that live in deep ocean water, though the biological process that produces the light in this group is unique. It's difficult to say what exactly the glow is for in this Phylum, but a number of purposes have been theorized. In addition to this bioluminescence, all comb jellies also show iridescence, the rainbow sheen that is so frequently seen when viewing them. The sheen is actually cause by light reflecting off the comb rows as they move, creating the colorful, flashing appearance.

Class Tentaculata

Class Nuda

There are two Classes in this phylum, and the division has been made based on whether or not the species posses tentacles at any point during their lifecycle. Species with tentacles are placed into Class Tentaculata, a group which includes the vast majority of comb jellies. All other species are placed into Class Nuda. Overall, species in Nuda have simpler body plans than those in Tentaculata. This includes the lack of oral lobes, which aid in prey capture as well as movement through the water. In addition, those tentacles seen in Tentaculata are used exclusively for prey capture. Nuda, in contrast, has developed a very stretchy mouth to help capture prey.

There is still much that isn't known about Ctenophores thanks to most species only being found in open ocean. Often, observations are chance encounters where individuals are caught in fishing nets. It isn't uncommon for scientific expeditions that are specifically looking to study these animals to discover new species during their searches.

Source is Biology of the Invertebrates. Images are from Wikimedia Commons and are under Creative Commons licenses or are copyright free: one, two, three, four, five.

Phylum Cnidaria

Moon jellyfish, one of the numerous species of cnidarian.

How a nematocyst fires

This phylum has quite a number of species included, but the most well-known of them is the group known as jellyfish. However, though all true jellyfish are cnidarians, not all cnidarians are jellyfish. though there are several characteristics unique to this phylum, one of them is post important to the general observer: the cnidae, of which the nematocysts are the most common. Nematocysts are stinging cells and they're what make brushing up against the wrong side of a jelly hurt so much. The nematocysts are contained within specialized cells called cnidocytes, which can be triggered in several ways, including direct contact or even just a change in water salinity. When the nematocysts are fired out, their barbs and spines will stick inside anything they come in contact with. One common action of the nematocycts is paralyzing the prey to make it easier to digest. Since cnidarians have soft, jelly-like bodies it's very easy for their body to be damaged, so it would be devastating for the animal to ingest struggling prey.

Cnidarians are a simple sort of animal, not even having a complete digestive tract. Their mouth also serves the same purpose as the anus found in other animals. This makes digestion someone inefficient since opening its mouth to ingest new food can cause the animal to loose a fair amount of partially digested food and any potential energy it could obtain from it. The do have a simple nerve net, muscle tissue, sensory organs, and numerous other things that the previously mentioned phyla do not have.

Medusa (left) and polyp (right)

This group has two general body plans: the medusa and the polyp. The medusa is the body form most associated with the jellyfish. The poly is what is seen in such creatures as coral and the hyrda. Many species have both, alternating between to body forms and also alternating between types of reproduction. While medusae are basically exclusively associated with sexual reproduction, polyps are capable of a number of things. Polyps in some species reproduce sexually, while others reproduce asexually through either budding or something called strobilation, and still other species may do all three. Strobilation, for the uninitiated, creates a number of genetically identical individuals (which will become medusae) at the mouth of the polyp is a sort of stacked-cup arrangement. In some species, the reproductive cycle goes something like this: medusa > sexual reproduction > ciliated larva (planula) > attached polyp > asexual budding > asexual strobilation > juvenile medusa (ephyra) > medusa. Reproduction is one of the major ways that cnidarians are classified.

There are a number of Classes within this phylum: Scyphozoa, Cubozoa, Hydrozoa, Myxozoa, and Anthozoa. Class Scyphozoa includes those jellyfish that virtually everyone knows about. They alternate between medusa and polyp, and one of their most distinguishing characteristics is their use of strobilation. Some in this group have zooxanthellae, or algae that live within the animal's tissue. Class Cubozoa is the box jellyfish, which are rather infamous for their painful sting. Their medusae are box-like and they also, believe it or not, have complex eyes with lenses. They have polyps and medusae, but do not strobilate. Class hydrozoa includes such species as the hyrda and unique colonials like the Portugese man-of-war. These species have a life cycle that is dominated by the polyp rather than the medusa. Class Myxozoa consists of strange parasites that I won't talk about for simplicity's sake. Class Anthozoa includes all of the corals and the anemones, polyp-only species that may be colonial (coral) or singular (anemones).

I could go on forever about the differences between they types and such, but I'm going to cut it short here.

Source is Biology of the Invertebrates. Images are from Wikimedia Commons and are under a Creative Commons license or are copyright free: one, two, three.

Phylum Placozoa

The placozoan: Trichoplax adhaerens.

There really isn't much to say about Phylum Placozoa. The phylum name is derived from the Greek for "flat plate animal," which is probably one of the best ways to describe these simple creatures. It currently consists of only one species, though the genetic diversity of that species is so great that it may very well be several. The species was first discovered in 1883, but, as is so often the case in Biology, it's a rather harmless species and thus is not well studied. These animals prefer marine habitats with shallow water, but they have also been found in aquariums. They are unusual among the animals due to being asymmetrical.

The body has only two layers of cells and there is no nervous system, digestive system, or even a body cavity. As with sponges, placozoans are able to reform into a single individual after being split apart. They are quite mobile and move not unlike an amoeba, even able to readily change shape. The largest individuals were produced in laboratories and are only a few millimeters across, while wild individuals are significantly smaller. As I already mentioned, there are only two cell layers. The ventral layer (i.e. the bottom layer) is flagellated and also contains cells that excrete digestive enzymes onto its food, which is then absorbed into the cells. This animal is so poorly studied that scientists have only guessed at its preferred diet: "it sits atop the algae and protozoans on which it apparently feeds." The dorsal layer (i.e. the upper layer) is thin and has only a few flagella. Placozoans reproduce asexually with ease, and do so regularly when under observation. Though sexual reproduction has occurred in a laboratory setting, very little is known about the process and the offspring produced have never developed past the sixty-four cell stage.

As for how these strange animals relate to other animals, it's difficult to say. Studies have variously supported and rejected placing them as basal or "primitive" animals, as a relative of sponges, and as a sort of simplified cnidarian. This is despite several different forms of DNA analysis.

Sources are Biology of the Invertebrates, University of California Museum of Paleontology, and Science 2.0. Image is from Wikimedia Commons under a Creative Commons license.

Phylum Proifera

A diver surveying orange finger sponges on a section of Grey's Reef National Marine Sanctuary off the Georgia coast.

One of the uncommon freshwater sponges

The name Porifera is Latin in origin and means "pore bearing." The use of this term for the phylum name is quite appropriate as these filter feeders are, indeed, covered in pores. They are aquatic, with the vast majority being marine. These are considered to be the simplest of animals; so simple in fact that they don't even have organs. They are quite unusual among animals since they lack symmetry, being amorphous with little rhyme or reason to their shape. Despite their simplicity, these animals are capable of some rather interesting feats. One experiment shows a sponge's ability to differentiate between its own cells and cells of another sponge. This was true even if the sponges were of the same species. Also, if the sponge is hacked apart or even put in a blender it is quite capable of reforming itself into a single sponge. Sponges play an important part in aquatic communities and are a common habitat for other animals, as well as other life such as bacteria. Though many sponges are dull, others can be quite brilliant in color.

A sponge with many oscula

These organisms have several specialized cell types. Choanocytes, also known as collar cells, are flagellated cells which move water through the body of the sponge and collect what food particles it can. Flat pinacocytes form the closest thing sponges have to skin: the pinacoderm. Archaeocytes are amoeba-like and digest food and perform whatever other task the sponge needs. This includes differentiating into other cell types, such as spongocytes, sclerocytes, and even sperm and egg. Porocytes are also seen, which are tube-shaped cells that surround the pores which serve as the entry point of water into the sponge. Oscula (singular: osculum), which are usually surrounded by pinacoderm rather than porocytes, serve as the exit point for the current of water created by the sponge.

A microscope view of silica spicules

Classification within the phylum is based on structural composition and morphology. All sponges produce protein fibers called spongin (produced by spongocytes), but not all produce the harder elements known as spicules (produced by sclerocytes). Spicules can be made up of either calcium carbonate or silica, and vary in shape and size. Some spicules are simple rods, while others may have several sections or be covered in tiny spikes. Spicules form an important protective layer in the dormant form that some sponges are able to produce, known as gemmules.

The three sponge body types

The three body types seen in sponges, listed in order of increasing complexity, are asconoid, syconoid,  and leuconoid. The addition of more complex twists and turns to the path that water has to travel through the sponge allows for much more efficient filtration. This is likely why the simple asconoid sponges are also the smallest and the complex leuconoid sponges are the largest. A large body requires more food, and more efficient filtering allows for the collection of more food.

Hexactinellid Venus's flower basket

The three classes in the Phylum Proifera are Class Calcarea, Class Demospongiae, and Class Hexactinellida. Calcarea includes all of the species with calcium carbonate spicules, as well as all of the species with the asconoid body type along with some synconoid and leuconoid sponges. Demospongiae is the largest group, consisting of species that are mostly leuconoid. Not all of the species in this class have spicules, and it is the only class to include freshwater species. The class name means "people's sponge" since bath sponges are included in the grouping. This group also contains the strange cladorhizid sponges which do not resemble other sponges and feed as carnivores. Hexactinellida is composed of species that always have complex silica spicules and as such they are also known as "glass sponges." They have many unique properties that have made some propose putting them in their own phylum.

It is commonly assumed that sponges form what could be called an "evolutionary dead end," since there are not any animals that appear to have come from sponges. Despite this, there is some molecular evidence that suggests the sponges with calcium carbonate spicules may have given rise to other animals.

Source is Biology of the Invertebrates. Images are from Wikimedia Commons and are under a Creative Commons license or are copyright free: one, two, three, four, five, six.

Animal Phyla: An Introduction

Image is from Wikimedia Commons under a Creative Commons license

Ashley over at Swamp Dog Blog has inspired me to give you readers a rundown of the animal phyla, and I mean all of the animal phyla. Or at least as complete and up-to-date as I can make it.

To begin, I would like to share a bit of a disappointment I have with America's education system. When you are taught science, and biology in particular you are under the assumption that what you are learning is all that there is. No more, no less. Even though the scientific method is taught as part of this, there is no real reference to change. The facts are the facts and you better learn them because otherwise you'll not know anything about biology. As such, for a student such as my younger self, the first time I heard of anything different from that perceived set of facts it was a bit startling. Now, perhaps that was a fault of the particular teachers I had, but the wonder that there is still so much not known about the world is part of the beauty of science. Yes, there are certain things that have been tested and tested and still hold true, but other aspects of science can be anything but concrete.

One example of this uncertainty that still remains in Biology is the classification system, or rather what organism goes in which grouping. Thanks to continued discovery of new species and new knowledge of the organism through DNA testing, things have become a bit of a mess. Not only do scientists disagree as to which classification system is better, but there is work toward completely overhauling some aspects. This includes the proposed complete restructuring of Eukaryotes that would replace the traditional Animal, Plant, Fungi, and Protist with anywhere from five to eight groupings. The restructuring makes since if you know anything about "protists," since the heading really has no meaning other then "stuff that can't be easily placed anywhere else." It's likely that this will filter down to high school level texts rather soon. When I was finishing my undergraduate studies, I tutored for a while and noticed that the basic biology texts already had this system included. Also, if you're curious, the proposed system would place animals under the heading "Opisthokont" along with Fungi and certain protistants. One nice thing about this proposed reorganization is that it wouldn't affect much if any of the lower order groupings such as phyla.

Anyway, back to animal phyla. One common misconception that students currently have about the animal world is that classification is simple and narrow. This is encouraged through the teaching of a simplified version of the classification. In fact, I was rather amused to come across a page that listed only nine animal phyla. Oh boy are there a lot more than nine! In fact, the text we used when I took Invertebrate Zoology listed the following phyla:

• Porifera
• Placozoa
• Cnidaria
• Ctenophora
• Platyhelminthes
• Mesozoa
• Rotifera
• Acanthocephala
• Gnathostomulida
• Micrognathozoa
• Nemertea
• Mollusca
• Annelida
• Arthropoda
• Tardigrada
• Onychophora
• Nematoda
• Nematomorpha
• Priapulida
• Kinorhyncha
• Loricifera
• Gastrotricha
• Chaetognatha
• Cycliophora
• Phoronida
• Brachiopoda
• Bryozoa
• Entoprocta
• Echinodermata
• Hemichordata
• Xenoturbellida
• Chordata

Of course, the text only talks about the non-vertebrate chordates, but vertebrates are such a comparatively small group that the book basically covers everything. So, that is where I will begin. I will present what is listed above, altering it as needed to express any new information that I find. There's some really weird stuff, so I hope you will enjoy reading about it!