ALCIPPE LAMPAS. 533 



closing the orifice, we can understand the necessity for the 

 defence afforded by the bands of spines and hairs on the 

 inner tunic of the sack and on the labrum, which do not 

 occur in other Cirripedes. 



Horny disc. — The general shape of the disc, its irregu- 

 larity and position, have been already described. It never 

 extends, as remarked by Mr. Hancock, to the extreme lower 

 point of the peduncle ; upwards it reaches to a little below 

 the lower end of the orifice. It consists of successive layers 

 of membrane, either moderately thick and opaque, or only a 

 little thicker than the general membrane of the body, but 

 never furnished with the little sharp points ; it increases in 

 size, in like manner as the calcareous valves of other Cirri- 

 pedes. the undermost and last formed layer extending beyond 

 the others, with its edge united, till the next exuviation, to 

 the general membrane of the body. The disc is attached, 

 at its upper end, apparently in the usual way, by cement, 

 to the roof of the cavity of the shell in which it is imbedded ; 

 but the lower parts of the disc are also slightly and partially 

 attached, chiefly along the lines of growth or exuviation ; 

 and this, I suspect, is effected by an inorganic calcareous 

 deposit ; anyhow I could not perceive here any cement or 

 cement-ducts. Beyond the circumference of the disc the 

 whole animal lies free in its cavity. The lines of growth in 

 the middle part of the disc are generally obliterated by the 

 decay of the older and outer layers. These lines, though of 

 course ordinarily conformable with the general outline of 

 the disc, are not always so, for the disc sometimes becomes 

 during growth slightly changed in form, and the animal, 

 consequently, slightly changed in position ; sometimes 

 either one or the other side or the upper end of the disc 

 is left deserted by the new layers of the growing disc ; 

 these being formed on the deserted side of less size or ex- 

 tension, instead of larger size, as they normally should be 

 all round the disc. 



The upper end of the disc is always produced into a 

 projection of not regular shape, but generally hollowed out 

 or embayed in front (fig. 1), and almost always hollowed 

 out on the two sides. This projection stands directly over 

 the adductor muscle (b in fig. 5), and on the exterior surface 



