ANNULOSA: CRUSTACEA. 201 



As regards the development of the Cirripedia, the larva is at 

 first oval in shape, devoid of segmentation, and protected by a 

 dorsal carapace. There is a single eye, two pairs of antennae, 

 and three pairs of natatory limbs, of which the two posterior 

 pairs are bifid at their extremities. The larva, in fact, in this 

 stage very closely resembles some of the smaller Entomostraca. 

 In a more advanced stage, the carapace becomes considerably 

 altered, the great antennae become modified for prehension, 

 the first pair of antennae (antennules) disappears, and the eye 

 becomes double. In a still more advanced condition (fig. 61), 



a 

 Fig. 61. Locomotive young of Balanus. a Eye ; b Caudal bristles ; c Setigerous limbs. 



the anterior pair of natatory limbs become bifid, like the pos- 

 terior ones; three additional pairs of limbs are developed 

 behind those already existing ; the abdomen becomes defined 

 from the thorax ; and the carapace is composed of two portions, 

 or valves, which enclose the thorax and abdomen with their 

 appendages. Finally, the young Cirripede fixes itself to some 

 solid object by means of the prehensile antennae. " The * ce- 

 ment-ducts ' can be traced as far as the third or ' disc-seg- 

 ment * of the antennas. There the cement seems to transude 

 and fasten down the disc ; soon both antennae are surrounded 

 by a common border of cement, which gradually increases in 

 extent after the metamorphosis. In the Lepas fascicularis the 

 cement is poured forth in sufficient quantities to form, itself, 

 the substance to which the peduncle of the adult barnacle 

 adheres, and for a cluster of which barnacles it constitutes a 

 central vesicular float." (Owen.) The cement-gland, as shown 

 by Darwin, is "part of, and continuous with, the branching 

 ovaria," and the cement-ducts open through the prehensile 

 antennae. 



The form of the adult, as already said, differs considerably, 

 but the two most important types are those presented respec- 

 tively by the Sessile and by the Pedunculated Cirripedia. 



In the common Acorn-shells (Balani y fig. 62, a) the anterior 

 portion of the head is not elongated, but is fixed to the centre 

 of a basal, membranous, or shelly plate, termed the " basis," 



