484 



KNTEROPNEUHTA. 



The body of the young Balaiioglossus (tig. 274 B) is divided into 

 three regions (1) the proboscidian region, (2) the collar, (3) the trunk 

 proper. The proboscidian region is formed by the elongation of the 

 prse-oral lobe into an oval body with the eye-spots at its extremity, and 

 provided with strong longitudinal muscles. The heart (Jit} and water- 

 vascular vesicle lie near its base, but the contractile cord connected 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 behind 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 cesopha- 

 geal region in the American one. 



The whole of the body, including the proboscis, becomes richly 

 ciliated. 



One of the most important characters of the adult Balanoglossus 

 consists in the presence of respiratory structures comparable with 

 the vertebrate gill slits. The earliest traces of these structures are 

 distinctly formed while the larva is still in the Tornaria condition, 

 as one pair of pouches from the oesophagus in the Mediterranean 

 species, and four pairs in the American one (fig. 275, br). 



In the Mediterranean Tornaria the 

 two pouches meet the skin dorsally, and 

 in the young Balanoglossus (fig. 274 B, br} 

 acquire an external opening on the dorsal 

 side. In the American species the first 

 four pouches are without external openings 

 till additional pouches have been formed. 

 Fresh gill pouches continue to be formed 

 both in the American and probably the 

 Mediterranean species, but the conversion 

 of the simple pouches into the complicated 

 gill structure of the adult has only been 

 studied by Agassiz (No. 568) in the Ameri- 

 can species. It would seem in the first 

 place that the structure of the adult gill 

 slits is much less complicated in the Ameri- 

 can than in the Mediterranean species. 

 The simple pouches of the young become 

 fairly numerous. The}^ are at first circular; 

 they then become elliptical, and the dorsal 

 wall of each slit becomes folded ; subse- 

 quently fresh folds are formed which 

 greatly increase the complexity of the gills. 

 The external openings are not acquired 

 till comparatively late. 



Our knowledge of the development of the internal organs, mainly 

 derived from Agassiz, is still imperfect. The vascular system appears early 

 in the form of a dorsal and a ventral vessel, both pointed, and apparently 



FIG. 275. LATE STAGE IN 

 THE DEVELOPMENT OF BALANO- 

 GLOSSUS WITH FOUR BRANCHIAL 

 CLEFTS. (After Alex. Agassiz.) 



m. mouth; cm. anus; br. 

 branchial cleft; Jit. heart; W. 

 water-vascular vesicle. 



