EARLY STAGES OF DEVELOPMENT OF BALANOGLOSSUS. 229 



Though a detailed comparison or general discussion of the 

 significance of these developmental facts cannot, of course, be 

 attempted until the later stages have been described, it may 

 perhaps be advisable to point out the chief features of difference 

 between the larvae just described and Tornaria. 



In the first place it will be seen that at no stage has this 

 larva any superficial resemblance whatever to a Tornaria pos- 

 sessing a longitudinal posterior band of cilia. This fact is the 

 more remarkable, as the adult appears closely to resemble 

 B. Kowalevskii, which is described by Agassiz as passing 

 through a Tornaria stage. 



At Stage H, however, the general contour of the body is 

 very like that of the late stage of Tornaria, and especially that 

 species described by Metschinkoff, in which a single pair of 

 gill slits is present. (In the Tornaria figured by Agassiz, in 

 this condition, four gill slits are already formed.) 



Now Tornaria (Metschnikoff, &c.) is a transparent larva, 

 possessing a prseoral band of cilia ; a longitudinal band of cilia, 

 and one or more transverse posterior bands of cilia, also a nearly 

 median water-vascular, archenteric diverticulum, opening to 

 the exterior a little to the left of the dorsal middle line. It 

 also has an apical epiblastic thickening, bearing a tuft of cilia 

 and a pair of eye-spots, from which a contractile string passes 

 to the inner end of the water-vessel. 



On the other hand, this larva is opaque; it has no prseoral 

 or longitudinal postoral bands of cilia, water-vascular system, 

 eye-spots, or contractile string, thereby differing markedly from 

 Tornaria. 



It resembles it in the possession of a transverse band of 

 cilia and an apical tuft of cilia. 



The opacity of this larva is however due to the presence of 

 food yolk in its tissues, and most of these points of divergence are 

 more or less such as might be expected to result from this fact, 

 consisting, as they do, chiefly in the absence of such a complete 

 apparatus for locomotion and [of the sense organs ; for it is 

 clear that an animal which passes a large part of its larval life 

 enclosed in an eggshell, and on emerging from it creeps in the 



VOL, XXIV. — NEW SER. Q 



