Protostome Dissection And Comparison Against Other Phyla Biology Essay


This report looks into the structure and function of some species in the Protostome Phyla Phylum Nematoda and Phylum Rotifera, and compares them against species from other Phylum (Cnidaria and Platyhelminthes). The Protosome Phyla contains roughly 30,000 different species living in a wide range of marine, freshwater and terrestrial environments (Hickman & Kats, 2009). The two Protosomes being studied, Ascaris and Rotifer belong to the Nematoda phyla and Rotifera phyla respectively. Ascaris and other species from the Nematoda phyla are commonly known as roundworms, due to their structure and shape and are generally parasitic organisms (101 Science, n.d). They possess a complete digestive system which can be considered a distinguishing feature of the Nematodes (101 Science, n.d). Nematodes are generally dioecious and only reproduce sexually, because of this a lot of their internal structures are designed to assist in the reproductive process (Hickman, 2009). Rotifer, like Ascaris feature a tube within a tube body structure, are dioecious, and have no respiratory or circulatory system (101 Science, n.d; Hickman, 2009). A distinguishing feature of the Rotifer is its crown (corona) of cilia on the anterior end. The shape of the crown looks similar to that of a wheel and is used for locomotion and feeding (101 Science, n.d; Hickman, 2009).

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Organisms from the Protostome Phyla which are studied in this report are considered pseudocoelomates. This means that they have a body cavity (called a pseudocoelom) located between endoderm and mesoderm. An organism possessing a pseudocoelom or a coelom is said to be more advanced, and have distinct advantages in comparison to acoelomates (Hickman & Kats, 2009). This lab will look further into the differing morphologies and the varying complexitites between the four organisms (Hydra, Rotifera, Planaria and Ascaris) and how each one functions.


See lab manual; 'Laboratory Studies in Animal Diversity fifth edition' (Hickman and Kats, 2009), pg 93 - 97.


Figure 1: Drawing of a Female Ascaris post dissection.

Figure 1 shows the interior and exterior structure of a female Ascaris one the dissection had been completed. Some structures like the oviducts and ovary where easy to indentify as they appear as multitude of long, thin, coil like tubes taking up a large percentage of the internal space. These connected to and wrapped around a thicker tube structure in the shape of a 'Y' located near the anterior end. This organelle is the uteri and vagina. The pharynx and following down intestine could be easily seen, and also at the opposing end the rectum. The anus could be seen but was hard to distinguish from the exterior. The other identifiable structure seen is the lateral line that runs up both sides of the body in the Ascaris.

Figure 2: Transverse section of a female Ascaris.

The large round/oval shape shown in Figure 2 is indentified as the uterus takes up a sizeable proportion of the body. Inside this structure eggs can clearly be seen. Five round balls clumped together on the left hand side of the slide are the oviducts. The lip shaped intestine can be clearly seen on the right hand side, easily distinguishable by its empty interior and flat posture. The long muscle cells can be seen running down both sides of the organisms and stopping where the lateral line (not visible) and excretory canal (not visible) would be. Pseudocoel is the white/clear substance that makes up the rest of the interior and supports the other structures. It is also possible to distinguish the thick cuticle from the thin epidermis around the outside of the speciemen.

Figure 3: Drawing of a live Rotifer.

Figure 3 shows some of the distinguishing features of the semi transparent Rotifer specimen. The ciliated corona is easily visible, and when moving looks like a rotating wheel. The mouth is located just in front of the corona. Due to its transparency the dark coloured brain is evident, located just below the corona and mouth area. The stomach takes up a large proportion of the body with salivary glands being visible in and/or behind it. At the bottom of the stomach the anus was distinguishable, as was the small pedal glands following down further. The toe visible is at the very end of the specimen.


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The dissection of the Ascaris and viewing a live Rotifer specimen it is clear that there are many differences between pseudocoelomate organisms from the Protosome Phyla and more basic organisms like Hydra or Planaria. The non segmented Ascaris that was dissected (Figure 1) was 30cm long. It can be classed as a psuedocoel as it has a body cavity (called a pseudocoelom) located between endoderm and mesoderm. This differs from a coelomate animal which has body cavity (called a coelom) is located within the mesoderm (see appendices 2) (Hickman & Kats, 2009). Ascaris can be said to fit the description of tube within a tube body construction. This relates to the structure of their internal organs all resembling or having tube like structure with a tube like external structure, with a separate opening an exit . In comparison a planarian doesn't fit this description as its acoelomate (Hickman & Kats, 2009). Cilium is present in both Cnidarians and Acoelomates, but absent from nematodes. There is no intermediate host in the life cycle of Ascaris. Once the Ascaris egg is ingested it stays in the host permanently. Sexual dimorphism describes species like the Ascaris, which have systematic differences between males and females of the same species (Hickman, 2009; Hickman & Kats, 2009). This is evident in that the male Ascaris are 15 to 30cm in length, while females are slightly larger at 20 to 40 cm long. Males have two chitinous spindles jutting out from the anal region and a curved posterior end (Hickman, 2009). In males the anus is a multipurpose opening, in that it both removes faeces, and is a genital opening. The other more blatant difference is that male's genital region differs to the female's vagina. Ascaris is transmitted from host to host through the passing of eggs in faeces, and those eggs being ingested by other organisms (Hickman, 2009; Hickman & Kats, 2009; Smyth & Wakelin, 1994). In countries where human faeces is used as fertiliser the problem is exaggerated. Infestation can be prevented by disposing of infected faeces in an efficient manner, for example the use of toilet facilities. Not using human or infected faeces as fertiliser, and washing hands thoroughly after going to the toilet, before touching food, and before eating. Ascaris has structurally and functionally adapted to life as a parasite in the intestine by having very elaborate reproductive organs which are associated with increased gamete production (Hickman, 2009; Hickman & Kats, 2009). This helps them produce up to 200,000 eggs a day. The body wall is covered by thick tough non-chitinous cuticle resistant to the host's digestive enzymes, but it is permeable to the salts and water. The cuticle is continuous with the cuticular lining of the pharynx foregut and rectum hindgut (Smyth & Wakelin, 1994).

It is evident through this study that species such as the Ascaris and Rotifer from the Protosome Phyla exert some major physiological advantages of more simple organisms such as the Hydra or Planaria (see appendices 1). This study has been highly successful in investigating and understanding the increase in complexity of the internal structures and functions in pseudocoelomate organisms.

Appendices 1




Platyhelminthes (Planaria)

Nematoda (Ascaris)


Radial or bi radial symmetry

Bilateral symmetry

Bilateral symmetry


Cylindrical body, mouth end has six to ten tentacles

Flattened body, sender, leaf like shape

Long, thin, tapered ends, tubule

Germ layers

Two embryological primary germ layers ectoderm, endoderm

Three germ layers, ectoderm, mesoderm, and endoderm

Three primary layers

Body covering

Thin layer of epidermal cells

Covering layers and two layers of muscles

Thick non cellular cuticle

Cephalization (present or absent)




Coelomic cavity (if present, state what type)




Musculature (layers present and how arranged)

Muscular contractions via epitheliomuscular cells, which form an out layer of longitudal fibres at the base of the epidermis and an inner layer of circular fibres at the base of the gastrodermis.

Primarily a sheath form of mesodermal origin; layers of circular longitudinal and sometime oblique fibres below the epidermis.

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Body wall has longitudinal muscles only

Digestive tract and digestion

Two types. Digestion within gastrovascular cavity is where extracellular digestion occurs. Food particles engulfed by cells of the gastrodermis, where intracellular digestion occurs

The digestive system has mouth, pharynx, and a gastrovascular cavity

Digestion system complete; muscular pharynx is triradiated in cross section


No anus or excretory system

No anus, waste excreted out of mouth

May include gland cells and canal, opens by excretory pore

Nervous system

Nerve net with symmetrical and asymmetrical synapses; diffuse conduction

Consisting of a pair of anterior ganglia with longitudinal nerve cords, connected by transverse nerves and are located in the mesenchyme

Ring of nervous tissue with ganglia around pharynx, dorsal and ventral nerve cords

Sense organs

Well developed statocysts and ocelli

Includes statocysts and ocelli

Include papillae, setae (for touch); anterior amphids and posterior amphids are likely chemoreceptors

Reproduction, sexual

Some species are monoecios and dioecious

Monoecious, sexual is main form of reproduction

Internal fertilization, sperm are amebord, not flagellated; cleavage pattern unique.

Reproduction, asexual


Transverse fission in some species chain of zooids


Appendices 2

Adapted from: (San Francisco State University, 1998)