354 BALANOGLOSSUS. 



of the heart in the proboscis, may have some excretory significance, but 

 it seems to be quite enclosed, {b) In the coUar-region, there are two 

 small paired coelomic cavities, from which two fiinnels open to the 

 exterior. Both these cavities and that of the proboscis tend to be ob- 

 literated 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. 



The Respiratory System consists of many pairs of ciliated gill-slits. 

 They open dorsally behind the collar, and the most anterior are slightly 

 overlapped by that structure. 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. Supporting the respiratory part of the gut there is 

 a delicate skeletal framework. 



The Vascular System includes a main dorsal blood-vessel lying above 

 the notochord, an anterior dilatation which is sometimes called the 

 " heart," a ventral vessel beneath the gut, and (in B. minutus at least) 

 two lateral vessels which receive branches from gut and gills. The 

 blood is said to flow forwards dorsally, backwards ventrally. 



The Excretory System is slightly developed, but from the region over- 

 lapped by the collar two ciliated funnels open to the exterior, and we 

 have already mentioned an enigmatical gland in the proboscis. 



Reproductive System. — The sexes are separate. A number of simple 

 paired genital organs lie in segmental order dorsally on each side of the 

 body-cavity in or behind the region with gill-shts. But their distribu- 

 tion does not appear to be quite constant. They open by minute dorsal 

 pores in the skin, or in the American species by rupture. 



Development. — The eggs must be fertilised outside of the body. Seg- 

 mentation is complete and approximately equal ; a blastosphere or 

 blastula results ; this is invaginated in the normal fashion, and becomes 

 a two-layered gastrula. 



Soon, however, a remarkable difference between different species is 

 manifest. The American species (B. kowalevskii) has a simpler de- 

 velopment than the others, for it is without a remarkable larval form 

 (Tornaria) which occurs in them. We shall take the simpler case first, 

 though it is perhaps less primitive. 



The blastopore or mouth of the gastrula narrows and closes ; the 

 external surface of the gastrula becomes ciliated ; the endoderm lies 

 as an independent closed sac within the ectoderm. Meanwhile the 

 embryo has become or is becoming free from the thin egg-envelope, and 

 begins to move about at the bottom in shallow water. It elongates and 

 becoines more worm-like ; there is an anterior tuft and a posterior ring 

 of cilia; the primitive gut forms five coelomic pouches ; a mouth and an 

 anus are formed, but there is no fore-gut nor hind-gut invagination ; 

 the regions of the body— the pre-oral proboscis, the collar, the gill-slit 

 region with two apertures to begin with, and the posterior region— are 

 defined at a very early stage. 



The Tornaria larva of other species is at first bell-shaped, with a 

 ventral mouth, curved gut, and posterior terminal anus, with external 



