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MANUAL OF ZOOLOGY. 



itself to some solid object by means of tlie prehensile antennae. 

 '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 antennas are surrounded 

 by a common border of cement, which gradually increases in 

 extent after the metamorphosis. In the Lepas fasciculctris 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 Dar- 

 win, is 'part of, and continuous with, the branching ovaria,' 

 and the cement-ducts open through the prehensile antennas. 



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. 



Fig. 50. Morphology of Cirripedia. a. Sessile Cirripede or Balanoid, Balanut 

 sulcatus. b. Pedunculate Cirripede or Lepadoid, Lepas anaiifera. 



In the common Acorn-shells (Balani,fig. 50, 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,' 

 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 

 shell itself and the operculum are composed of calcareous 

 plates, usually differing from one another in shape, and dis- 

 tinguished by special names. Within the shell the animal is 

 fixed, head downwards. The thoracic segments, six in num- 

 ber, bear six pairs of limbs, each of which consists of a jointed 

 protopodite and a much segmented exopodite and endopo- 

 dite, both of which are ciliated, and constitute the so-called 

 ' cirri,' from which the name of the sub-class is derived. These 

 twenty-four cirri the ' glass hand ' of the Balanus are in 

 incessant action, being protruded from the opening of the 

 shell, and again retracted within it, constantly producing cur- 



