To investigate the next step in the development of the hydrostatic skeleton obtain a living specimen of a nematode worm. The Phylum Nematoda is included with a group of aquatic phyla known collectively as the pseudocoelomates. These animals have a body cavity (which developed from the embryonic blastocoel) that is not lined with mesoderm, and are enclosed in a tough cuticle composed chiefly of the fibrous protein, collagen. Watch your nematode through the microscope. Notice its seemingly mindless thrashing? We'll see in a minute that it actually does move quite effectively, but first let's investigate the nematode on a structural and mechanical basis.

(A) Pseudoceolomate body cavity

Obtain a cross section of Ascaris, an internal parasite of mammals (Ascaris, by the way, is an extremely large nematode - most aremuch smaller). Notice first that Ascaris has a large body cavity, the pseudocoel, that contains the flattened gut and various reproductive organs (why do you think there is such an emphasis on reproduction in this roundworm?). The pseudocoel is enclosed by a body wall containing longitudinally-arranged muscles that are divided into four "fields" by the projections of ventral and dorsal nerve cords and lateral cords that contain excretory canals. In life the pseudocoel is filled with pseudocoelomic fluid that is maintained under positive pressure by the muscular body wall and the cuticle that maintains the nematode's round shape. If you poked a hole in a living Ascaris the pseudocoelomic fluid would squirt out! After you have found your way around the Ascaris slide, dissect a preserved specimen and find the parts of the animal listed above.

(B) Movement using fields of muscles

Structurally the stage is set for a hydrostatic skeleton - a non- compressible fluid enclosed and pressurized in a body cavity....but hold it! Something's missing! We've only described one set of muscles, the longitudinal muscles. Where are the expected antagonistic circular muscles? The answer is there are no circular muscles. Instead there are the four "fields" of longitudinal muscles, which Ascaris can contract in groups producing local shortening. Because the pseudocoelomic fluid is incompressible, the internal pressure increases causing stretching of muscle cells in another part of the body. Through this system of local contractions of the muscle fields the dorsal and ventral longitudinal musculature act as antagonists, producing sinusoidal waves along the length of the nematode's body. Most nematodes lie on their sides and the resulting dorsi-ventral undulations move the nematode in the horizontal plane through an aquatic medium. Now again watch your nematode moving. Can you see the sinusoidal undulations? What role might the stretchable cuticle play in this movement? Can you think of any limitations to this movement? What about the relationship between the size of the nematode and the viscosity of the medium?

Now, if you were to improve on the nematode hydrostatic skeleton what would you add? Although the sideways thrashing ultimately moves the nematode along, locomotion probably could be improved significantly. How about adding the missing antagonistic circular muscle layer? How about taking the pseudocoel and lining it with mesoderm on both sides so the gut is now supported by mesenteries and provided with its own musculature? On top of that let's take our newly developed body cavity, now called a true coelom, and separate it into distinct segments that can be closed off from one another to compartmentalize the coelomic fluid. We've just described an annelid worm, a member of the Phylum Annelida.