CIRRIPEDIA. 291 



the pre-existing thorax of the pupa, but within that part of the pupa 

 (homologically a portion of the first three cephalic segments) which 

 lies anterior to the thorax. As a consequence of this, the pedicels 

 and lower portions of the cirri, the segments of the thorax and its 

 dorsal surface, all come to occupy a position at nearly right angles 

 to that of the corresponding parts in the pupa. And, as a further 

 consequence (and this is a more important point), the sack, which both 

 in the young Cirripede and pupa is formed by the overhanging 

 and produced portion of the carapace, and which is internally lined 

 by a reduplication of the membrane of the thorax, is necessarily, 

 owing to the changed position of the thorax, altered in extent and 

 carried much further ; namely, from extending merely parallel to the 

 longitudinal axis of the pupa, it is now, in the young Cirripede, in 

 addition, carried almost quite across the inside of the animal. Hence 

 it comes that the young Cirripede is internally almost intersected ; 

 and its body remains attached only by a small space to the sternal 

 or ventral inner surface of the carapace, — the carapace being modified 

 either into the capitulum and peduncle, or into the shell with its 

 operculum and basis. As a still further consequence of this change 

 of position of the body of the young Cirripede within the body of 

 the pupa, the alimentary canal becomes shortened to fully half its 

 former length. At the same time, the interspace between the mouth 

 and first pair of legs of the pupa (consisting of the seventh and 

 eighth segments of the archetype) is quite lost in the Cirripede by 

 coalescence. The first cause of the thorax of the young Cirripede 

 not being developed within the thorax of the pupa, probably is, that 

 the cirri may be formed of considerable length, so as to be immediately 

 able to seize prey ; and that the thorax, which supports the cirri 

 (and this probably is even more important), should be as free as 

 possible within the sack, so as to aid the captorial action of the 

 cirri." * 



When the due time for the act of metamorphosis has arrived, the 

 pupal carapace splits along the dorsal ridge, and is cast off together 

 with the acoustic sacks, the basal segments of the two antennae, and 

 the great black compound eyes. " Besides the split along the dorsal 

 ridge, the carapace separates all round the orifice from the delicate 

 tunic lining the sack and investing the thorax and natatory legs of the 

 pupa ; for those membranes are not moulted for some considerable 

 time afterwards. Hence all these inner parts retain for a period the 

 appearance and structure of the natatory pupa, whilst the exterior 

 resembles in every respect a fixed and perfect cirripede." f 



* CCXXIV. p. 123. t lb. p. 126. 



u 2 



