39° 



HEMICHORDA OR ENTEROPNEUSTA. 



consisting of five distinct parts, all of which are lined by 

 mesoderm, and arise as pouches from the primitive gut or 

 archenteron. (a) There is first the unpaired cavity of the 

 proboscis, which communicates with the exterior by a dorsal 

 pore (or sometimes 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 significance, but it seems to be quite enclosed. 



(b)In the collar region 

 there are two small 

 paired ccelomic cavi- 

 ties, from which two 

 funnels open to the 

 exterior. Both these 

 cavities and that of 

 the proboscis tend 

 to be obliterated by 

 growth of connective 

 tissue, (c) Two other 

 cavities extend along 

 the posterior region 

 of the body, to some 

 extent separated by 

 the dorsal and ventral 

 mesentery which 

 moors the intestine. 

 In these there is a 

 body cavity fluid with 

 cells. 



Respiratory and 

 vascular systems. — 

 The respiratory 

 system consists of many pairs of ciliated gill-slits. They 

 open dorsally by small pores behind" the collar. In develop- 

 ment they begin as a pair, increase in number from in front 

 backwards, and they go on increasing long after the adult 

 structure has been attained. Water passes in by the mouth 

 and out by the gill-slits, where it washes branches of the 

 dorsal blood vessel. There are no gill lamellae associated 

 with the slits. 



The vascular system includes a main dorsal blood vessel, 



Fig. 170. — Transverse section through gill- 

 slit region of Ptychodera minuta. — After 

 Spengel. 



The section, somewhat diagrammatic, shows a gill- 

 slit (^.tf.) to left, and a septum between two slits 

 to the right; d.n., dorsal nerve; d.v., dorsal 

 vessel; v.n., ventral nerve ; v.v. t ventral vessel ; 

 ^., nutritive part of gut ; £"!., respiratory part of 

 gut; c, lateral ccelomic spaces ; l.m., longitud- 

 inal muscles; R., reproductive organs. As the 

 gill-slits_ are oblique, the whole of one could not 

 be seen in a single cross-section. 



