DESCRIPTION OF BALANOGLOSSUS. 395 



cance, but it seems to be quite enclosed, (b) In the collar 

 region there are two small paired coelomic cavities, from 

 which two funnels open to the 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 sep- 

 arated by the dorsal and ventral mesentery which moors the 

 intestine. In these there is a body cavity fluid with cells. 



Respiratory System. 



The respiratory system consists of many pairs of ciliated 

 gill slits. They open dorsally by small 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 vessels. There are no 

 gill lamellae associated with the slits. 



Vascular System. 



The vascular system includes a main dorsal blood vessel, 

 which, at its anterior end, lies above the notochord ; an 

 anterior dilatation, which is sometimes called the " heart ; " 

 a ventral vessel beneath the gut; and numerous smaller 

 vessels. The blood flows forwards dorsally, backwards 

 ventrally. This system should be contrasted with that of 

 Amphioxus. 



Excretory System. 



The excretory system is slightly developed. 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. 



Reproductive System. 



The sexes are separate. A number of simple paired 

 genital organs lie dorsally in a series on each side of the 

 body cavity in and behind the region with gill slits 

 (Fig. 125, R}. They open by minute dorsal pores in the 

 skin, or in the American species by rupture. 



