212 MANUAI, OF ZOOLOGY. 



nse) are transformed into prehensile organs ; the two following 

 pairs of limbs are cast off; and six pairs of strong, biramose, 

 natatory feet are developed upon the abdomen. A pair of 

 composite eyes are present, but there is no mouth. Finally, the 

 pupae fix themselves by the prehensile antennas to rocks, drift- 

 wood, ships, Sponges, Cetaceans, Turtles, Crustaceans, or even 



Fig. 64. — Locomotive "pupa" oi Balanus. a Eye; b caudal bristles; 

 c Setigerous limbs. 



Jelly-fish. The prehensile antennae are glued down perman- 

 ently by the secretion of a peculiar cement-gland. The cara- 

 pace becomes, as a rule, the seat of definite calcifications, by 

 which, it is converted into a multivalve calcareous "test;" the 

 mouth is developed, and the six pairs of natatory feet are con- 

 verted into long jointed " cirri," by which food is conveyed to 

 the mouth. " The ' cement-ducts ' can be traced as far as the 

 third or ' disc-segment ' of the antennae. There the cement 

 seems to transude and fasten down the disc ; soon both anten- 

 nae 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 bar- 

 nacles it constitutes a central vesicular float." — (Owen.) The 

 cement-gland, as shown by Darwin, is " part of, and continu- 

 ous with, the branching ovaria," and the cement-ducts open 

 through the preh^sile antennse. 



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, fig. 65, «) 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," 

 which adheres by its external surface to some solid body. 

 Above the basis rises a more or less limpet-shaped, or conical, 

 shell, which is open at the top, but is capable of being com- 

 pletely closed by a pyramidal lid, or " operculum." Both the 



