290 LECTURE XIII. 



In the third or pupal stage^, the anterior natatory legs become, like 

 the others, bifid or biramous : and three posterior pairs of legs are 

 superadded ; the small abdomen becomes defined from the thorax. 

 The prehensile antennre are freed from their cases ; the two eyes 

 stand further apart. The carapace consists of two portions, like tlie 

 valves of a bivalve shell, which fold over and include the thorax and 

 abdomen and their appendages, permitting also the prehensile an- 

 tennas {^fig. 126, a) to be bent back within the lower edge. The tw^o 

 valves of the shell are drawn together by an adductor muscle, which 

 crosses anterior to the basal margin of the mouth. Darwin has 

 observed the pupa of the Lepas australis to " swim very rapidly, and 

 often on one side, in a circle ; it could walk by the aid of its antennas, 

 but it often fell over." The antennce are the organs by which the 

 young Cirriped finally anchors itself to the spot where its future 

 adult existence is to be spent. The cement-ducts may be traced as 

 far as the third or disc-segment of the antennoe. There the cement 

 seems to transude, and to fasten down the disc : soon both antenna3 

 are surrounded by a common border of cement, which gradually 

 increases in extent after the metamorphosis. In the Lepas faseicu- 

 laris, the cement is poured out 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, as may be seen in the preparations, Nos. 275 and 276, Nat. 

 Hist. Series, Hunterian Museum. 



The three terminal segments of the antennae, into which the 

 cement-ducts are prolonged, are retained in an otherwise functionless 

 condition, in the young Cirriped. The mouth is formed under that 

 of the pupa, with a new oesophagus round the old oesophagus, leading 

 into the same alimentary canal. The twenty-four extreme tips of 

 the six pairs of biramous cirri of the young Cirriped are formed 

 within the twenty- four extremities of the six pairs of biramous 

 natatory legs of the pupa. " Consequently," writes Darwin, " in 

 the Cirripede and pupa, thus far, part corresponds with part, not- 

 withstanding that new eyes are formed posteriorly to the old eyes, 

 and new acoustic organs in a quite different position from the old 

 ones ; but now we come to a most important diversity in the meta- 

 morphosis, or rather, to follow Professor Owen *, in the metagenesis, 

 of the young Cirripede. Although, as just stated, the extremities of 

 the cirri are formed within the legs of the pupa, yet, from the great 

 length of the cirri, they occupy more than the whole of the thorax of the 

 pupa : so that the thorax of the young Cirripede is not formed within 



* Parthenogenesis, pp. 13 and 26. 



