CIEEIPEDIA. 



439 



like shell composed of several (4, 5 or more) pieces. These pieces, 

 which originate by the deposition of calcareous matter in the chi- 

 tinous covering of a large reduplicature of the skin (mantle), are 

 distinguished as scuta, terga, and carina. The animal is invariably 

 fixed by the anterior end of the head, which in the Lepadidce, (fig. 

 348, a) may be drawn out into a long stalk projecting freely from 

 the shell. In the Balanidce, which are without the stalk (fig. 348, 5), 

 the body is surrounded by an external calcareous tube, usually com- 

 posed of six pieces ; the aperture of the tube is closed by a sort of 

 operculum formed of calcareous plates lying inside (fig. 348, b). In 

 Te 



FIG. 348 a, Lepas after removal of the right shell. A', Anterior antennae at the end of the 

 stalk ; C, carina ; Te, tergum ; Sc, scutum ; Mk, oral cone ; F,. caudal fork ; P* cirrus or 

 penis ; M, muscle, b, Balanut tintinnabulum (after Ch. Darwin), one-half of the shell has 

 been removed; Tu, Section of the outer shell ; Oo, ovary; Od, oviduct; Oe, opening of 

 oviduct ; Ad, adductor muscle ; Sc, scutum ; Te, tergum ; A', anterior antenna. 



both cases the attachment is effected principally by the hardening of 

 the secretion of the so-called cement gland, which opens on the 

 penultimate joint of the small and delicate anterior antennae; this 

 joint being dilated to form a sort of sucker. The body, which is 

 surrounded by the mantle and its shell-plates, lies with its hinder 

 region stretched upwards so that the appendages, which are used to 

 cause currents in the water, may be protruded from the slit -like 

 space left on the ventral side between the paired scuta and terga. 

 Appendages and external features. A head with antennae and 



