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Kingdom Animalia is the superset of a wide variety of life. On the extremes of diversity (near the “edges”) there are organisms with largely differentiated body plans such as the segmented organisms (Arthropods) and those with (visually) minimal body plans - like the clam. The development of different body plans represents a major historical evolutionary shift in the development of Kingdom Animalia and the atmospheric circulatory system of the biosphere.

In order to identify the mechanism which has historically allowed organisms to diversify in their growth we look at animal embryonic development. Embryonic development would not be possible without sexual reproduction. The growth and development of a particular organism starts with fertilization of an egg by a gamete. The progression of developing structures in this fertilized egg is dependent on the species of animal (the hereditary information in the form of DNA). While plants possess a characteristic identified as the alternation of generations animals do not. Instead animals rely on embryonic development. All animals develop from a zygote to a blastula and then to a gastrula. The gastrula provides a “cavity”. The importance of the cavity in the gastrula is that of the specific structure of sponges. Sponges are specifically developed as if embryonic development was “frozen” at the stage at which the gastrula is formed. The development of the gastrula shows how it goes into more complex organism. How does that stomach with a mouth get further in the jellyfish? The body plan has got to change. The Cnidarians have radial symmetry. The first major evolution in the body plan is going to be when this form (the hydra form) begins to elongate and flatten. As it elongates and flattens, and takes a horizontal or vertical approach, then we get bilateral symmetry.

The specialization tending towards bilaterally symmetrical organisms leads to the speciation of worms. The study of embryonic development points out that the fertilized egg grows into larger and important structures which in turn leads to sponges, jellies, hydra, and now worms. The constant throughout the process is that of symmetrical structures. The earthworm has a bilaterally symmetrical body plan. This means that if you were to look at the organism in the proper view that you can cut the view from the “top” to the “bottom” of the screen (down the middle) and have two parts of the organism that basically “match” each other.

With the development of bilaterally symmetrical body plans comes the next major evolutionary step especially in flatworm embryonic development. This is the formation of acoelomates or organisms with no body cavity. Most importantly, the gastrula begins to differentiate between the cell layers in the flatworms. In sponges and cnidarians it is either “inside or outside”. In flatworms, we have organ systems beginning to arise from the cell layers (endoderm, ectoderm, and mesoderm). Mesodermal layer is beginning in the flatworm. This is the structural evidence for the flatness of the worm - no internal structural support exists. The flatworm itself is like a ribbon. It is difficult to tell which side is the head and which side is the tail (one-way digestive tract). The eyespots on the worm indicate the head. Furthermore the existence of the eyespots indicates the presence of cephalization. Cephalization is the evolution of sensory organs in the anterior end of an organism. (We see a great deal of cephalization in the crawdad.)

The pattern of cephalization does not stop with worms. Even in the mollusks we have studied there is indication of cephalization. Cephalization has proven itself over the years to be an important process and could not occur without nervous tissues. Sponges have a single type of tissue. Jellyfish have more than one. Mollusks definitely have more than one, and Arthropods are no different in this context.

The second advancement in the evolution of animal embryonic development is the formation of a pseudocoelom. The worm phyla are important because of the evolution of acoelomate (no body cavity) to a pseudocoloemate, which has a fluid-filled body cavity, to a coelomate, which has an air-filled body cavity. (Pronounced “see-lo-mate” = coelomate).

Pseudocoelomates are roundworms and nematodes. The major body plan innovation in roundworms, such as this nematode, is the presence of a body cavity. You see the mouth, the presence of the mouth attached to a digestive tract that now has an exit. There exists more than one opening. To a roundworm, it means that it can continue to eat. The flatworms, what they do, when they feed them, they excrete digestive juices, liquefy the food, pull it into the digestive cavity, transfers it to the entire cell network, and excretes the waste from the same cavity out the pharynx. The flatworm is limited to eating only after it is completely processed any nutrients taken in. The roundworms are the first group of organisms with a mouth and an anus, a one-way digestive tract. That's important.

Secondly, roundworms have true muscles. They only have longitudinal muscles. Most of them were found in our examples of pond water. “This thing is spastic,” some students said (while looking at the pond water), and whipping around, is a classic S-shaped movement of all roundworms. Roundworms do not have circular muscles. The S-shape is forced due to the structure and lack of internal support.

Thirdly, the third thing that you see is that there is no opening in the mouth-to-anus. The muscles are outside of that, the mesoderm. In between the mesoderm and the digestive cavity is a fluid-filled space. Here's why this is important. When the muscles move, it forces food through the digestive track. Imagine having to run 2 miles after eating. We do not have to have that. The hydrostatic pressure of the fluid forces it on to the digestive tract, causing the materials to move down. Some advantages of fluid-filled body cavities? Protection. Throw it against a water-bed, it bounces off. Secondly, it provides a semi-skeletal structure (hydrostatic skeleton) (not bone). Roundworms are round because of the pressure from the inside and outside pressure sources. It also makes them pop when you step on them but that's a different matter entirely.

Roundworms, especially nematodes, are more than likely the second greatest number of terrestrial organisms there are. Some scientists think that if we can go out and count a square foot of fertile farm land that there are more than 1,000,000 nematodes.

The mollusks are related to all of this worm-development and indeed we also studied the mollusks. This group is the annelids. These are the true segmented organism. They have circular muscles and longitudinal muscles. It allows them to move in 2 directions at once. They have an air-filled body cavity. They are capable of moving in two directions at once, or in other words there is no requirement of absolutely linear muscle movements. Air-filled body cavities divorce locomotion from digestion. Waves of digestion are possible without locomotion … leading to many more potential behaviors unrestricted by feeding habits.

This is the story of evolution. We start with a sponge, we close the holes, call it a jellyfish, flatten the holes, call it a flatworm, but all of them basically have no body cavity. The nematodes have a water-filled body cavity. The coelom is an air-filled body cavity. Nematodes have pseudocoeloms. The annelids have a coelom.

The main difference (and the important one) between pseudocoelomates and coelomates is that nematodes have to move to digest. Coelomic animals do not have to move to digest. This was a major evolutionary step. The digestive motions are removed from the locomotion motions.

Why does the evolution of phylum Mollusca do away with the shells? The shells were particularly hazardous to survival in situations where movement and flexibility is required. This was a significant problem in the path of the survival of Mollusks. The problem with all of this is the next step of evolution for nematodes really involved both of those organisms (Annelids and Mollusks) at the same time. It branched off quickly. This serves as an example of where a group of species changes considerably to produce something different. For example we can see that the physiology of the clam seems to be similar to that of worms (except that clams secrete their shells among other differences).

Perhaps more important in this lab report is the relation of the simpler physiological formations that were discussed with human systems. The evolutionary path of animal embryonic development shows us that it is the human systems that are more complex and require more embryonic development than a large number of other animals. For example, see the time it takes for the human embryo to develop versus simpler animal embryonic development - the human takes nearly an entire year for development. Throughout the rest of this lab report there is a focus on tissues, bilaterally symmetrical body plans, and physiology of the specimens in relation to human physiology for analogy and comparison to better assimilate the wide variety and large collection of information.

The materials that persisted throughout the dissections were basic dissection tools: trays, scissors, needles, and pins. Pins are used to effectively immobilize the prey while cutting and snipping away. Needles serve as pins and as a fine-point probe. Scissors allow for incisions and the tray serves as a place to keep the organic material, unfortunately not the stench.

The dissections usually were executed in some manner by cutting vertically from the anterior end to the posterior end (or sometimes posterior to anterior - whichever was easier to start with). By making a vertical incision the epidermis can be peeled back and the internal organ structure becomes visualized. Usually there is a layer of muscle directly under the skin. The crawdad was an example of this. After cutting off the exoskeleton there were gills to deal with. The gills were easily removed, and after removing them we realized that the gills were “encased” in a cavity-like structure, but nevertheless the removal of the gills did provide for illumination of the internal anatomical structural visualization required to proceed with the dissection.

In general we worked “outwards” to “inwards”. We recursively removed large layers of tissues (organs) until we got to the “center” at which point we stopped and disposed of the specimen. By going inward we were able to better preserve the structures than by vicious mutilation (say, stabbing the specimen on the spot and tearing it open). Animal embryonic development starts with a few core cells which then grow outward by increasing the number of cells in the formation (zygote to blastula to gastrula and so on). Interestingly, we were working in the reverse of that process: starting from the final structure and moving inwards.

The majority of information and data collected in these dissections was qualitative observational information rather than quantified numerical information. Perhaps with the assistance of a stronger understanding of genetics and the complete genome of the specimens, along with a bloody fast computer, numerical information could be extracted. However in the meantime the qualitative and opinionated observations perhaps hindered by paradigm will have to suffice.

The earthworm was segmented. There were about 15 segments from the clitellum to the anterior (head-)end. We started the dissection by inserting scissors into the belly-side of the earthworm. We then cut upwards towards the anterior (head)-end. We were able to tell that it was the head end because of the small, black eyespots. After cutting up to the nearly last segment the skin was pulled to the left and right and then was pinned down.

The skin itself was not precisely “smooth” but rather rough. Supposedly this would be useful for friction generation (traction) while in dirt. On the other hand, a large portion of the worm had smooth skin. The smoother skin was not near the head, suggesting that the worms which have a rougher head are able to dig and the worms with smoother ends are able to slide them right along. (This matches our conventional wisdom of things despite our inability to [naturally] burrow through dirt like a worm.)

The bivalves have two (“bi”) “valves” - or two shell parts. The anterior end (towards the backside where the “hinge” would be) has “teeth like” projections that support the clam as it closes and opens. Without this structure the clam would likely collapse on itself when opening and closing. When the valves were opened, it was observed that the external shell felt rough while the internal portion of the shell felt much smoother and damp/moist.

One of the questions that arises with the clam is that of locomotion. The clam has no specific locomotive capacities when viewed from the outside. However, despite this lack of external locomotive capacity clams are found all over the world. This suggests that they are either 1) susceptible to ocean currents, 2) common food, and/or 3) able to move on their own through a different way. Indeed, the clam can be moved by ocean currents, and the clam is hunted by some birds (see the birds which drop the clams from high in the sky). However, during the dissection the siphon was found. The siphon is an important structure for the movement and persistence of homeostasis in the clam. The siphon inhales water and then exhales it (though not immediately). This action allows the clam to eat, too. The water is washed over the gills. The rough portion of the gills can capture food particles. The smoother portions of the gills do not generate as much friction against the food particles, however, they can still be used for respiration.

The squid was kept moist before we got to it. There were 10 tentacles attached. Some sources say that these are not all tentacles though the majority of them were supposedly still the tentacles. Attached to the tentacles were many tiny “suction cups” supposedly advantageous in that they can be used to capture food. Between the eyes on the ventral side was the siphon which took incoming water which would then go through the mantle and come out the “exhalent” siphon (output only). This is supposedly how the squid is able to make quick movements.

The mantle was not “flimsy” like the mantle cavity of the clam which we had to cut away to enter the “soup” of organs. In the squid there was more organization. It was generally easier to see what organs were what (unlike in the clam).

The crayfish had an exoskeleton formed on the upper portion of the body. There were 10 appendages, making it a decapod. Once the exoskeleton was removed the gills could be seen. The gills were like “slices of meat”. When the walking legs were moved back and forth, the gills would also move back and forth. This suggests that the crayfish has greater oxygen intake while in rapid motion. The swimmerets on the underside of the tail looked like they would be used for swimming guidance. The appendages near the area of the tail seemed to be unspecialized, hosting muscle and the intestine (leading to the anus near the uropods).

The dissection called for the removal of the mandibles. The mandibles were difficult to locate and remove. They were shaped like small “blobs” of skin with no definite shape (except the 3^rd maxilliped). The heart was found immediately under the carapace (exoskeleton). The brain was, strangely unlike in previous dissections, attached to the back-side of the exoskeleton at the anterior end, near the green glands. Another interesting characteristic was that of the position of the ventral nerve cord, which was on the ventral (belly) side of the organism, unlike, say, in Homo sapien sapiens, where it is on the back-side (dorsal) near the spinal skeleton. The ventral nerve cord was close to the legs, suggesting that it was evolutionarily advantageous for the species to have possibly faster neuronal input transmission. Bringing the nerve tissue together allows for shorter propagation times and quicker reflexes.

The study of different organisms under the scalpel can lead to important conclusions concerning the development of animals throughout past history. Not all animals display the same type of symmetry. The bilaterally symmetrical organisms have a near “mirrored” image of everything. The radially symmetrical organisms are generally smaller than the larger species of the bilaterally symmetrical animals (suggesting a size restriction/boundary).

The development of the acoelomate, the pseudocoelomate, and the coelomate was also important. The acoelomate has no dy cavity. The coelomate has an air-filled body cavity. The pseudocoelomate has a fluid-filled body cavity. The pseudocoelomates that I have studied have had open circulatory systems. The coelomates that I have studied have had closed circulatory systems. Air mixing with blood in a system is usually not good - even humans are killed when too much oxygen enters the bloodstreams.

Along with bilaterally symmetrical organisms came the process of cephalization, the development of neuronal perception structures (eyes, texture sensors, other various organs, like the crawdad's balance organ). The crayfish is a good example of an organism which has undergone (and still is undergoing) cephalization. The crayfish, as a decapod, has 10 appendages each with nervous tissue throughout … to process the vast amount of information there has to be a sufficient brain, however in the crawdad this is strangely absent. The squid is an example of an organism that has developed a larger brain to account for the available information from the nervous tissue.

Throughout these dissections there has been a subtle focus on the homeostatic processes in each organism. The questions that have risen up are those of: how does the organism supply oxygen to the cells? What does it eat? What structure aides the organism in gathering and processing food? What does the animal do with food? Where does the food go? What does the animal do to supply the cells of its body with oxygen and food? What is the relationship between the similar human systems?