278 



LECTURE Xm. 



it expands at once into a broad disc or basis of adhesion ; in both 

 cases the immediate attaching substance is a peculiar cement pro- 

 ceeding from a modified portion of the ovaria,* The typical Cirripeds 

 are divided according to these modes of attachment into two primary 

 groups, viz., the pedunculated, or Lepadoids, and the sessile, or Bala- 

 noids. The first are commonly known by the name of Barnacles ; the 

 second by that of Crown-shells or Acorn-shells. Such are the 

 characters of the typical members of the class. The aberrant bur- 

 rowing genus Alcippe, and the naked, ventless Proteolepas, para- 

 sitic on other Cirripeds, form, according to Darwin, types of two 

 orders, equivalent respectively to that including all the ordinary 

 Cirripeds. 



Most of these Cirripeds have their visceral cavity protected by a 

 calcareous shell composed of many pieces ; but in some, as, e.g. Alepas, 

 Ibla, the membranous investment of the viscera is enveloped by 

 a smooth elastic sheath, continued from the outer chitinous covering 

 of the peduncle. In Otion two small calcareous bodies (scuta), de- 

 veloped in the substance of the outer envelope, just above the brachial 

 fissure, are usually the sole rudiments of a shell, and the chitinous 

 covering is produced at its free extremity into two cylindrical plicated 

 ear-like appendages. In the earless Otions {Cineras), the external 

 tunic is strengthened by five calcareous bars, two at the fissure {scuta), 

 giving outlet to the arms, two along the terminal 

 margin of the tunic (terga), and one along the 

 dorsal aspect {carina). In the common Barnacle 

 {Lepas anatifera), the calcareous matter extends 

 from five homologous centres, so as to protect the 

 whole of the visceral or thoracic-abdominal ca- 

 vity or body. In Jig. 122, c is the " scutum," d 

 the " tergum " and e the " carina ; " the first two 

 are in pairs, the last is single, and was compared 

 by Cuvier to the symmetrical dorsal valve of 

 the Pholas ; but the relation is one of analogy 

 only. 



All the valves, however, are strongly marked 

 with lines of growth, formed by successive ad- 

 ditions to their margins, as in the shells of 

 Mollusca. In the genera Pollicipes and Scalpellum, there are other 

 smaller calcareous plates arranged round the junction of the body 

 •with the peduncle. In^^f. 122, a and 6 are the peduncle, which is a 

 development of the head including the aborted eyes and anteunte of 



Lepas anatifera. 



* CCXXIII. p. 33. 



