THEIR RANK. 13 



apparently arises from the fixed condition of the animals of 

 both groups. 



The carapace, which covers the dorsal surface of the 

 larva in the first stage, in the last larval or pupal stage is 

 developed so as to enclose, like a bivalve shell, the whole 

 body. In the mature Cirripede, the whole external cover- 

 ing, whether shell and operculum, or capitulum and pedun- 

 cle, can be conclusively shown to be the carapace of the 

 pupa, modified. In thus enclosing the mouth and whole 

 body, the modified carapace resembles that of several Ento- 

 mostracans ; but in being apparently formed (as I hope 

 hereafter to show) by the development of the third segment 

 of the head, and in consisting generally of distinct sclero- 

 dermic plates, arranged in an imbricated order, there is, 

 I think, a closer resemblance to the same part in some of 

 the Podophthalmia. The carapace, or portions of the cara- 

 pace, being capable of other movements, besides merely 

 opening and shutting, differs, I believe, from that of all other 

 Crustaceans ; as it likewise does* in the greater part not 

 being periodically moulted. 



Moreover, in all Cirripedes there is another striking 

 peculiarity connected with these parts, namely, the exclusive 

 attachment of the whole thorax or included body to the 

 internal ventral or sternal surface of the carapace and head. 

 In the pupa, the thorax, as in all Crustaceans, opens into, 

 and is continuously united with, the large anterior part of 

 the head ; but from the singular fact that the thorax of the 

 young Cirripede is developed not within the thorax, but 

 within the head of the pupa (PI. 30, fig. 2), with its lon- 

 gitudinal axis placed at right angles to that which it held in 

 the pupal condition (the mouth and the whole exterior being 

 developed conformably with that of the pupa), it comes to 

 pass after the metamorphosis, that the Cirripede is, as it 

 were, internally cut in twain (compare PI. 25, fig. I, and 

 PI. 30, figs. 2 and 3). Thus it is, as will hereafter be more 

 fully explained, that the sack originates, and thus the body 

 becomes attached to the internal ventral surface of the 

 carapace and front of head. 



* The carapace, however, of the Isaura, a Branchiopod, according to M. Joly 

 (' Amialcs des Sc. Nat.,' 2 ser. vol. xvii, p. 293), is not moulted. 



