582 I'N I'KUOl'NKUSTA. 



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 American 

 species. It would seem in the first place that the structure of 

 the adult gill slits is much less complicated in the American than 

 in the Mediterranean species. The simple pouches of the young 

 become fairly numerous. They are at first circular ; they then 

 become elliptical, and the dorsal wall of each slit becomes folded ; 

 subsequently 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 

 ending blindly at their two extremities. The two spurs of the water-vascular 

 vesicle, which in the Tornaria stage rested upon the stomach, now grow 

 round the oesophagus, and form an anterior vascular ring, which Agassiz 

 describes as becoming connected with the heart, though it still communicates 

 with the exterior by the dorsal pore and seems to become connected with the 

 remainder of the vascular system. According to Spengel (No. 572) the 

 dorsal vessel becomes connected with the heart, which remains through life 

 in the proboscis : the cavity of the water-vascular vesicle forms the cavity of 

 the proboscis in the adult, and its pore remains as a dorsal (not, as usually 

 stated, ventral) pore leading to the exterior. 



The eye-spots disappear. 



Tornaria is a very interesting larval form, since it is inter- 

 mediate in structure between the larva of an Echinoderm and 

 trochosphere type common to the Mollusca, Chxtopoda, etc. 

 The shape of the body especially the form of the ventral 

 depression, the character of the longitudinal ciliated band, the 

 structure and derivation of the water-vascular vesicle, and the 



