394 



HEMICHORDA OR ENTEROPNEUSTA. 



"chitinous" skeleton, which gives the sht a U shape on 

 account of the growth downwards of a " tongue bar," the 

 whole is suggestive of At?iphioxus. Beneath the branchial 

 skeleton there lies a "chitinous" rod, which divides into 

 two in the collar. 



The Body Cavity. 



The body cavity is somewhat complex, consisting of five 

 distinct parts, all of which are lined by mesoderm, and 

 arise as pouches from the primitive gut or archenteron. 



Fig. 125. — Transverse section through gill slit region of 

 Ptychodei-a ininida. (After Spengel.) 



The section, somewhat diagrammatic, shows a gill slit (^.f.) to 

 left, and a septum between two slits to the right; d.n.^ dorsal 

 nerve ; d.z'., dorsal vessel ; v.n., ventral nerve ; v.i'., ventral vessel ; 

 ^., nutritive part of gut; ^'. , respirator^' part of gut; c, lateral 

 ccelomic spaces; /.?«., longitudinal muscles; A'., reproductive 

 organs. As the gill slits are oblique, the whole of one could not be 

 seen on a single cross section. 



(a) There is first the unpaired cavity of the proboscis, which 

 communicates with the exterior by a dorsal pore (or some- 

 times by two) at the base of the proboscis next the collar. 

 It is possible that a glandular structure, which lies in front 

 of the heart in the proboscis, may have excretory signifi- 



