METAMORPHOSES OF CIRRIPEDES. 131 



tends to draw down the upper or posterior end of the 

 animal, now forming the opercular valves ; and as the 

 basis soon becomes throughout cemented to the surface 

 of attachment, the young Cirripede is much depressed. 

 Soon the opercular valves are drawn a little way down 

 within the shell, becoming attached to the sheath, instead 

 of, as at first, to the very summits of the compartments. In 

 regard to the changes which take place in the shell, in the 

 number of the segments in the cirri, and in the number of 

 spines borne on these segments, &c, during the continued 

 growth of the animal, as they are chiefly important for the 

 identification of the species, I will here refer to a discussion 

 on this subject under the head of the Genus Balanus. 



On the Homologies of the Carapace and Shelly Valves. — 

 In the pupa, the carapace is produced, not only posteriorly, 

 but anteriorly, so as to cover the entire animal, with the 

 exception of a narrow sternal surface (PI. 30, fig. 4) : in 

 front it is notched, where the sternal surface terminates, 

 and from this notch a faint line runs along the dorsal sur- 

 face, separating its tergal elements. In the young Cirri- 

 pede, after the metamorphosis, there is no trace of this 

 medial dorsal suture, or of the wider sternal surface. 

 Looking at the several genera of the Lepadidae, the ex- 

 ternal covering of the whole peduncle and capitulum is so 

 continuous and of so uniform a nature, that I think we 

 must consider the whole as a carapace, of which the sternal 

 borders have become completely confluent ; formerly I was 

 inclined to look at the capitulum alone as formed by the 

 carapace, and at the peduncle as being composed of the 

 two or three anterior cephalic segments, cased only by their 

 own integuments. As far as can be discerned, the cara- 

 pace in the pupa, and consequently in the Cirripede, con- 

 sists only of the tergal elements of the segments ; and this 

 seems likewise to be the case with the carapace of the 

 Podophthalmia. Until lately,* Prof. Milne Edwards doubted 

 whether the carapace in the higher Crustaceans (to which 

 I believe the carapace of Cirripedes must be compared) was 

 formed by the backward production of the third segment, 



* Compare ' Histoire Naturellc des Crustaces,' torn, i, p. 27, with ' Annates 

 des Sciences Nat./ 3d series, torn xvi, 1851, p. 2133. 



