4i8 



SUB- PHYLUM HEMICHORDA. 



l.m 



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 minute 

 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. 



The vascular sys- 

 tem includes a main 

 dorsal blood vessel, 

 which, at its anterior 

 end, lies above the 



notochord, and below a separate contractile dilatation, some- 

 times called the " pericardium " ; a ventral vessel beneath the 

 gut ; and numerous smaller vessels. The almost colourless 

 blood flows forwards dorsally, backwards ventrally. This 

 system should be contrasted with that of Amphioxus. 



Excretory and reproductive systems. No nephridia are 

 known, but from the region of the collar two ciliated funnels 

 open to the exterior, and we have already mentioned the 

 enigmatical proboscis gland. 



v.n. 



FIG. 206. Transverse section through gill- 

 slit region of Ptychodera minuta. After 

 Spengel. 



The section, somewhat diagrammatic, shows a gill- 

 slit (g:s.) 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., ventral vessel ; 

 g., nutritive part of gut ; g 1 ., respiratory part of 

 gut; c., lateral coelomic 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. 



