218 CRUSTACEA. 



While these processes of growth are taking place, the whole 

 thorax undergoes a significant change of position (Fig. 106 A and B). 

 Whereas, in the earlier stages, the thorax lay almost parallel to the 

 ventral surface, it now rises into a more perpendicular position with 

 regard to that surface, so that the anal aperture no longer lies behind 

 the oral aperture, but above it. Simultaneously with this change of 

 position, a sharper distinction arises between the thorax and the 

 cephalic region, the point (x) at which the wall of the thorax unites 

 with the surface of the mantle shifting ventrally (cf. the position of 

 x in Fig. 106 A, with its position in Fig. 106 B). 



Meanwhile many changes take place in the anterior cephalic region 

 of the pupa, these being preparatory to its transformation into the 

 stalk of the adult form. The broad basal joint of the adhering 

 antenna first completely fuses with the head and is taken up into 

 the latter, so that the adhesive antennae of the adult Cirripede each 

 consist of only three joints. Further, somewhat behind this point, 

 a deep infolding of the surface of the body occurs (Fig. 106 13, y), so 

 that that portion of the head which forms the stalk is at this stage 

 sharply bent on itself. 



This fold arises by the withdrawal of the stalk-integument from the cuticle of 

 the Cypris larva. Some of the important organs of the larva, however, which 

 are not to be taken over into the adult, remain attached to that cuticle. The 

 chief of these are the paired eyes (the Nauplius eye, however, passes over into 

 the adult Cirripede and is retained throughout life), and the chitinous processes 

 called by DARWIN the apodemes, which served for the attachment of the 

 antennal muscles and are cast off in the moult which follows. These details are 

 not represented in the figure. 



The most important change which now follows is in the position 

 of the larva and the lengthening of the stalk which is connected 

 with it. By an opening out of the fold above mentioned, the larva 

 now rises from the surface of attachment, its ventral surface standing 

 at right angles to the latter. At the same time the stalk passes out 

 from between the shell- valves of the Cypris stage (Fig. 106 C), and 

 lengthens to form the adult peduncle. 



DARWIN (No. 40) pointed out the fact that the part by means of which the 

 attachment of the larva at first takes place does not correspond to the frontal 

 extremity of the body, but to the most anterior portion of the ventral surface. 

 Only after the upright position has been assumed does the frontal extremity 

 come into contact with the surface of attachment, to which it is glued by a 

 secretion. In Crytophialus, on the contrary, and also in Alcippe, Lithotrya, and 

 Anelasma, this part is not firmly attached, but grows out further. This further 

 growth can only take place when the substratum can give way correspondingly. 



