METAMORPHOSES OF CIRRIPEDES. Ill 



I 



nature of which I apprehend no objection will be raised. 

 In two out of the three orders into which Cirripedes may 

 be divided, the mouth is succeeded, in the adult animal, by 

 eleven most distinct segments ; of which the first (i. e. the 

 seventh cephalic) differs from the succeeding seven thoracic 

 segments ; and these seven again differ from the three ab- 

 dominal and terminal segments. Hence it must be ad- 

 mitted that, as far as the cephalo-thorax of the archetype 

 Cirripede is concerned, it consists, like that of the archetype 

 Crustacean, of fourteen segments, of which eight succeed the 

 first-named six that form the mouth and front of the head ; 

 and that, with the three abdominal segments, there are alto- 

 gether seventeen segments. In the order Thoracica, however, 

 which includes all common Cirripedes, both in the pupa and 

 in the mature animal, only six thoracic segments with their 

 appendages, succeed the mouth, two having been lost ; and 

 the question arises which are these two, whether the seventh 

 and eighth, or the thirteenth and fourteenth (i. e. the two ter- 

 minal thoracic) segments; for there is no reason to suspect any 

 other segments of having disappeared. In my former volume, 

 I inferred, without sufficiently entering into my reasons, that 

 it was the seventh and eighth, i. e. the last cephalic and first 

 thoracic segments, which had disappeared ; but I now find 

 that Mr. Dana* believes that, in ordinary Crustaceans, the 

 abortion of the segments with their appendages almost 

 always takes place at the posterior end of the cephalo- 

 thorax. Nevertheless, after due deliberation and fresh exa- 

 mination of the pupa, I must retain my former opinion, 

 that it is the last cephalic and first thoracic segments which 

 have either coalesced with the others, or wholly disappeared. 

 In the pupa, the mouth, although functionless, has its place 

 most plainly marked by being slightly prominent, and by 

 the presence of a sort of labrum and of a shrivelled oeso- 

 phagus, round which latter the gnathites and the new oeso- 

 phagus of the future young cirripede are in process of 

 formation. Now between the mouth of the pupa and the 

 first pair of natatory legs, there is a space of membrane, 

 equalling, when stretched out, the three succeeding thoracic 

 segments in length and breadth : this interspace, I conceive, 



* ' Crustacea : United States Exploring Expedition,' p. 22. 



