The somatic layer of the trunk somites becomes converted into the musculature of the body wall and the external peritoneal layer of body cavity. The first part of the muscular system to be definitely formed is the ventral band of longitudinal muscles which arises on each side of the nervous system in contact with the epidermis ([fig. 157], m). How the circular muscles become subsequently formed outside these muscles has not been made out.
The splanchnic layer of the trunk somites gives rise to the muscular and connective-tissue wall of the mesenteron, and also to the walls of the vascular trunks. The ventral vessel is first formed (Kowalevsky) as a solid mass of cells which subsequently becomes hollowed out. The dorsal vessel in Lumbricus and Criodrilus is stated by Kowalevsky and Vejdovsky to be formed by the coalescence of two lateral vessels; a peculiarity which is probably to be explained by the late extension of the mesoblast into the dorsal region.
The layer from which the sacks for the setæ and the segmental organs spring is still doubtful. The sacks for the setæ are believed by Kowalevsky (No. [342]) to be epiblastic invaginations, but are stated by Hatschek (No. [339]) to be mesoblastic products. For the development of the segmental organs the reader is referred to the chapter on the excretory system.
In marine Polychæta the generative organs are no doubt mesoblastic products, as they usually spring from the peritoneal epithelium, especially the parts of it covering the vascular trunks.
The Alimentary Canal. In Lumbricus the enteric cavity is formed during the gastrula stage. In Criodrilus the hypoblast has at first no lumen, but this becomes very soon established. In Euaxes on the other hand, where there is a true epibolic gastrula, the mesenteron is at first represented by a solid mass of yolk (i.e. hypoblast) cells. As the central amongst these become absorbed a cavity is formed. The protoplasm of the yolk cells which line this cavity unites into a continuous polynuclear layer containing at intervals masses of yolk. These masses become gradually absorbed, and the protoplasmic wall of the mesenteron then breaks up into a cylindrical glandular epithelium similar to that of the other types.
In Lumbricus and Criodrilus the blastopore remains as the mouth, but in Euaxes a new mouth or rather stomodæum is formed by an epiblastic invagination between the front end of the two mesoblastic bands. This epiblastic invagination forms the permanent œsophagus; and in Lumbricus trapezoides and Criodrilus, where the oral opening is at first lined by hypoblast, the epiblast soon becomes inflected so as to line the œsophageal region. The splanchnic mesoblast of the cephalic region subsequently invests the œsophagus, and some of its cells penetrating between the adjoining epiblast cells give rise to a thick wall for this part of the alimentary tract; the original epiblast cells being reduced to a thin membrane. This mesoblastic wall is sharply separated from the muscular wall outside, which is also formed of splanchnic mesoblast.
The anus is a late formation.
Alternations of generations.
Amongst Chætopoda a considerable number of forms exhibit the phenomenon of alternations of generations, which in the same general way as in the case of the Cœlenterata, is secondarily caused by budding or fission.
The process of fission essentially consists in the division of a parent form into two zooids by the formation of a zone of fission between two old rings, which becomes differentiated (1) into an anal zone in front which forms the anal region of the anterior zooid, and (2) into a cephalic zone behind which forms the head and some of the succeeding segments of the posterior zooid. The anal zone is capable, by growth and successive segmentation, of giving rise to an indefinite number of fresh segments.