of a few Annelids. 245 



is quite a delicate grey, of a yellowish tinge, with a bright ycllov/ 

 line along the middle of the back and following the outhne of 

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

 black pigment-spots which are well developed on the head and first 

 ring and attain 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 in Polydora is quite different from that in Leucodora, 

 and from that observed by Claparede and by myself in Nerine : 

 in the latter they are more abundant and intense in the anterior 

 extremity ; while in Polydora they take their maximum develop- 

 ment from the middle of the body towards the posterior part, 

 leaving the anterior extremity, with the exception of the head, 

 nearly colourless. 



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

 ening of the lips, into an ill-defined oesophagus which extends 

 to the fifth ring and then opens 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 pigment- 

 spots round the eyes, and their increase on the four anterior 

 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 

 bunch of thin bristles, two hook-shaped bristles instead of one; 

 the oesophagus is more clearly marked than in the preceding 

 stage ; the glands are found in all the rings except the last. In 

 the following stage (fig. 28) there has been a still further 

 growth of the tentacles, and we find the pigment-spots arranged 

 both above and below in four parallel rows, the outer rows 

 being the smaller and less prominent. There are now four 

 hook-shai)ed bristles at the base of each of the dorsal cirri, 

 which are readily recognized as such in the rings immediately 



Ann, §' Mag. N. Hist. Ser. 3. Vol. xix. 18 



