BALANOGLOSSUS 



493 



the unpaired cavity of the proboscis, which communicates 

 with the exterior by a dorsal pore at the base of the pro- 

 boscis next the collar, (b) In the collar region there are 

 two small paired coelomic cavities, 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. 



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Fig. 280. — Transverse section through gill-sht region of 

 Ptychodera minuta. — After Spengel. 



The section, somewhat diagrammatic, shows a gUl-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 ; gi., respiratory part of gut ; 

 c, lateral coelomic spaces ; l.m., longitudinal muscles ; R., 

 reproductive organs. As the gill-slits are oblique, the whole of 

 one could not be seen in a single cross-section. 



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 

 development 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 is a main dorsal blood vessel, which, at its anterior 



