In the Chætopoda and the Tracheata the body cavity arises as a series of paired compartments in the somites of mesoblast ([fig. 350]) which have at first a very restricted extension on the ventral side of the body, but eventually extend dorsalwards and ventralwards till each cavity is a half circle investing the alimentary tract; on the dorsal side the walls separating the two half cavities usually remain as the dorsal mesentery, while ventrally they are in most instances absorbed. The transverse walls, separating the successive compartments of the body cavity, generally become more or less perforated.
Chordata. In the Chordata the primitive body cavity is either directly formed from a pair of alimentary diverticula (Cephalochorda) ([fig. 3]) or as a pair of spaces in the mesoblastic plates of the two sides of the body ([fig. 20]).
As already explained (pp. [294]-300) the walls of the dorsal sections of the primitive body cavity soon become separated from those of the ventral, and becoming segmented constitute the muscle plates, while the cavity within them becomes obliterated: they are dealt with in a separate chapter. The ventral part of the primitive cavity alone constitutes the permanent body cavity.
The primitive body cavity in the lower Vertebrata is at first continued forwards into the region of the head, but on the formation of the visceral clefts the cephalic section of the body cavity becomes divided into a series of separate compartments. Subsequently these sections of the body cavity become obliterated; and, since their walls give rise to muscles, they may probably be looked upon as equivalent to the dorsal sections of the body cavity in the trunk, and will be treated of in connection with the muscular system.
Fig. 351. Section through the trunk of a Scyllium embryo slightly younger than 28 f.
sp.c. spinal canal; W. white matter of spinal cord; pr. posterior nerve-roots; ch. notochord; x. subnotochordal rod; ao. aorta; mp. muscle-plate; mp´. inner layer of muscle-plate already converted into muscles; Vr. rudiment of vertebral body; st. segmental tube; sd. segmental duct; sp.v. spiral valve; v. subintestinal vein; p.o. primitive generative cells.
As a result of its mode of origin the body cavity in the trunk is at first divided into two lateral halves; and part of the mesoblast lining it soon becomes distinguished as a special layer of epithelium, known as the peritoneal epithelium, of which the part bounding the outer wall forms the somatic layer, and that bounding the inner wall the splanchnic layer. Between the two splanchnic layers is placed the gut. On the ventral side, in the region of the permanent gut, the two halves of the body cavity soon coalesce, the septum between them becoming absorbed, and the splanchnic layers of epithelium of the two sides uniting at the ventral side of the gut, and the somatic layers at the median ventral line of the body wall ([fig. 351]).
In the lower Vertebrata the body cavity is originally present even in the postanal region of the trunk, but usually atrophies early, frequently before the two halves coalesce.
On the dorsal side of the gut the two halves of the body cavity never coalesce, but eventually the splanchnic layers of epithelium of the two sides, together with a thin layer of interposed mesoblast, form a delicate membrane, known as the mesentery, which suspends the gut from the dorsal wall of the body ([figs. 119] and [351]). On the dorsal side the epithelium lining of the body cavity is usually more columnar than elsewhere ([fig. 351]), and its cells partly form a covering for the generative organs, and partly give rise to the primitive germinal cells. This part of the epithelium is often known as the germinal epithelium.