260 M. R. Leuckart on the Young States of some Annelides. 



characters, to ascertain the pedigree of our larva, or at all events 

 its nearest allies. These, in my opinion, are to be found in the 

 family of the Aricm, and indeed in the group with two long 

 tentacular cirri {Aricm Naidece, Oerst.; Sjjioidea, Grube), pro- 

 bably amongst the species of the genus Spio or Nerine. 



Before setting forth the reasons in favour of my supposition, 

 we must first of all get a good notion of the larva itself ; espe- 

 cially as Busch^s statements do not take up every point, and are 

 also erroneous in this respect, that he treats the dorsal surface 

 of the worm as the ventral surface, and vice versa. 



The most developed individuals that occurred to me (PI. VII. 

 fig. 1) measured 2^ lines and were of a brownish colour. They 

 were rather slender, about /^^'^^ ^^ ^ ^^^^ i^ width anteriorly, 

 tapering towards the hinder extremity and flattened on the back, 

 so that the height of the body (fig. 3) was scarcely more than 

 half its breadth. They swam, at least so it appeared to me, 

 almost exclusively by the action of the circlets and rows of cilia, 

 the arrangement of which has already been described by Busch. 

 As soon as they were interrupted in their movements, they rolled 

 themselves up into a flat spiral and sank to the bottom. 



The segments of the body (fig. 1) had already attained a con- 

 siderable number (about fifty). They exhibit a very uniform 

 structure, but gradually decrease in development from before 

 backwards. The head consists of a short, broad, nearly qua- 

 drangular tubercle, which is but indistinctly separated from the 

 first segment of the body, and bears, between the rounded ante- 

 rior angles, a short tentaculiform process (frontal tentacle) of 

 a conical form. The upper surface of the head is furnished with 

 four black ocelli placed at a considerable distance from the 

 median line, and standing in a line which is but slightly curved 

 backwards. The posterior and inner eyes are the largest, and 

 are sometimes divided on one side into two spots placed close 

 together. No refractive media could be found in them. At each 

 side of the head there is a very considerable tentacle (a tenta- 

 cular cirrus) which arises from the angle between the head and 

 the protuberant upper lip, and curves backwards like a horn 

 over the first segment. Where this meets the upper lip, the 

 latter bears a tuft of long cilia, which Busch has regarded, cer- 

 tainly correctly, as the remains of the crown of cilia. 



The anterior segment of the body is produced forwards at the 

 ventral surface almost to the base of the frontal tentacle (fig. 2), 

 and here forms a lip-like protuberance in front of the mouth ; this 

 is the upper lip already mentioned, which at the same time marks 

 the boundary between the buccal segment and the head. Even 

 without this process the first segment of the body is the longest, 

 although on the back it only measures about one-third of its 



