202 MANUAL OF ZOOLOGY. 



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 corn- 



Fig. 62. Morphology of Cirripedia. a Sessile Cirripede or Balanoid, Balanus sulcatus. 

 b Pedunculate Cirripede or Lepadoid, Lepas anatifera. 



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- 

 rents of water, and thus bringing food to the animal. There 

 are no specialised respiratory organs in the family of the 

 Balanidce. 



In the Barnacles (Lepadidcz, fig. 62, b) the anterior ex- 

 tremity of the animal is enormously elongated, forming with 

 the prehensile antennae, the cement-ducts, and their exuda- 

 tion, a long stalk or peduncle, whereby the animal is attached 

 to some solid object. At its free extremity the peduncle bears 

 the " capitulum," which corresponds to the shell of the Ba- 

 lanoids, and is composed of various calcareous plates, united 

 together by a membrane, moved upon one another by appro- 

 priate muscles, and protecting in their interior the body of the 

 animal with its appendages. The thorax and limbs resemble 

 those of the Balanus ; but " slender appendages, which from 

 their position and connections are homologous with the 

 branchiae of the higher Crustacea, are attached to, or near to, 

 the bases of a greater or less number of the thoracic feet, and 



