On the Young Stages of a few Annelids. 327 



head ; this is rendered mure brilliant by its contrast with the 

 black pigment spots well developed on the head and first ring, 

 and taking their greatest prominence on the sixth, seventh, and 

 eighth, gradually diminishing to a few isolated dots near the 

 anus. On the head it is almost impossible to distinguish the 

 eyes proper from the pigment spots ; it is, however, evident 

 that the eyes are more numerous in the young than in the 

 adult, which is the case with many other Annelids as observed 

 by Milne Edwards, Agassiz, and Claparede. The identity of 

 the pigment spots and eyes has been suggested by Claparede, 

 who could discover no difference between them, and we have 

 perhaps, in the pigment spots scattered over the whole surface 

 of the body, something analogous to the presence of eyes in 

 Fabricia at the anal extremity. The distribution of the pigment 

 spots of Polydora is quite different from that of Leucodora, 

 observed by Claparede and by myself in Xerine ; in the latter 

 they are more abundant and intense in the anterior extremity, 

 while in Polydora they take their maximum development from 

 the middle of the body towards the posterior part, leaving the 

 anterior extremity, with the exception of the head, nearly 

 colorless. 



The mouth opens by a longitudinal slit formed by the thick- 

 ening of the lips, into an ill-defined oesophagus extending to 

 the fifth ring, and then reopening into a digestive cavity ter- 

 minating at the anus, and not yet divided into a true stomach 

 and intestine. 



In the next stage, Fig. 27, we find no material change in the 

 anterior part, with the exception of the slight increase in 

 length of the tentacles, the diminution in number of the pig- 

 ment spots round the eyes, and their increase on the four an- 

 terior rings. The posterior part has considerably increased in 

 size, a number of additional rings having been formed in front 

 of the anal ring; the pigment spots are now arranged in two 

 regular rows, the dorsal cirri have not increased in size, but we 

 find in the seventh and succeeding rings, at the base of the 



