BALANOGLOSSUS. 



437 



Eespiratory and vascular systems. The respirator) 

 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 



R- 



FIG. 235. Transverse section through gill-slit region of 

 Ptychodera minuta. After Spengel. 



The section, somewhat diagrammatic, shows a gill-slit Cf.J.) to left, 

 and a septum between two slits to the right ; </.., dorsal 

 nerve; d,v., dorsal vessel; v.n., ventral nerve; v.v., ventral 

 vessel ; g:, nutritive part of gut ; .!, respiratory part of gut ; 

 ., lateral coelomic spaces; /.*#., longitudinal muscles; /?., 

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

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



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 

 end, forms a heart lying above the notochord, and below 

 a closed contractile dilatation, sometimes -called the " peri- 

 cardium." Beside. the latter there is a paired "proboscis 

 gland," formed from the ccelomic epithelium. There is 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 



