ENTEROPNEUSTA. 



5 8l 



a double-walled cylinder round the alimentary tract, but their 

 cavities remain separated by a dorsal and ventral septum. 



Eventually (Spengel) the cavity of the anterior cylinder 

 forms the section of the body cavity in the collar of the adult, 

 and that of the posterior (fig. 274 B, c] the remainder of the 

 body cavity. The septa, separating the two halves of each, 

 remain as dorsal and ventral mesenteries. 



The conversion of Tornaria (fig. 274 A) into Balanoglossus 

 (fig. 274 B) is effected in a few hours, and consists mainly in 

 certain changes in configuration, and in the disappearance of 

 the longitudinal ciliated band. 



The body of the young Balanoglossus (fig. 274 B) is divided 

 into three regions (i) the proboscidian region, (2) the collar, 

 (3) the trunk proper. The proboscidian region is formed by the 

 elongation of the prae-oral lobe into an oval body with the eye- 

 spots at its extremity, and provided with strong longitudinal 

 muscles. The heart (hi] and water-vascular vesicle lie near its 

 base, but the contractile cord con- 

 nected with the latter is no longer 

 present. The mouth is placed on 

 the ventral side at the base of the 

 prae-oral lobe, and immediately be- 

 hind it is the collar. The remainder 

 of the body is more or less conical, 

 and is still girt with the larval 

 transverse ciliated band, which lies 

 in the middle of the gastric region 

 in the Mediterranean species, but 

 in the cesophageal region in the 

 American one. 



The whole of the body, including 

 the proboscis, becomes richly cili- 

 ated. 



One of the most important cha- 

 racters of the adult Balanoglossus 

 consists in the presence of respira- 

 tory structures comparable with the 

 vertebrate gill slits. The earliest traces of these structures 

 are distinctly formed while the larva is still in the Tornaria 



FIG. 275. LATE STAGE IN THE 

 DEVELOPMENT OF BALANOGLOS- 

 SUS WITH FOUR BRANCHIAL 

 CLEFTS. (After Alex. Agassiz.) 



in. mouth ; an. anus; br. bran- 

 chial cleft ; ht. heart ; W. water- 

 vascular vesicle. 



