STUDIES ON POLYCHEHT LARVA. 617- 
band of powerful cilia further back on the body. In the larva 
of Telepsavus costarum, described by Claparéde.and 
Mecznikow (1869, pp. 178-181, Pl. XIV, figs. 1-1#), the 
number of segments situated in front of the mesotroch will 
be found to correspond to the number of segments. of the 
anterior region of the adult (see Claparéde, 1868, Pl. XX, fig. 
1) ; thus the precise position at which the functional secondary 
ciliated band is situated upon the body—this band having 
been rendered necessary by the suppression of the prototroch 
on account of the reduction of the size of the prostomium— 
would appear to be determined by some influence correlating 
larval and adult characters. 
We may even venture further than this, for in the Cheetop- 
terus larva, whose development has been described by 
Béraneck (1894), two mesotrochs are present; these he 
found to delimit one definitive segment, and this segment 
proved to be the first one of the second body-region, i.e. the 
segment which in this form is produced laterally into a pair 
of wing-like processes. And probably it is the differentia- 
tion of this segment from the others of the second region 
that has decided the position of the posterior ciliated band, 
just-as the differentiation of the two first regions appears to 
have decided the position of the anterior band. That the 
posterior band may have been acquired more recently than 
the anterior is suggested by its appearance at a later stage 
in the ontogeny than the anterior band (see Fewkes, 1885, 
p- 180, Pl. III, figs. 16-18 for ? Phyllochetopterus, and 
Gravely, 1909, for? Chetopterus). It is unfortunately im- 
possible in this connection to speak with certainty of the larva 
of Chetopterus variopedatus (pergamentaceus) de- 
scribed by Wilson. His oldest larve (twelve days) give no 
clue to the relation between the larval and adult body-regions, 
and in addition leaves quite obscure the homologies of the 
two ciliated bands which he describes. The cilia of both 
these bands at first graduate into those covering the general 
surface ; the anterior band is extremely transitory, disap- 
pearing just as the posterior band is beginning to develop, 
