ALCIPPE LAMPAS. 551 



very soon to form a great bag extending beyond the 

 attached prehensile antennae. Now if we suppose an analo- 

 gous structure in the female or ordinary Alcippe, and the 

 supposition is quite allowable, we shall almost immediately 

 have the anterior or lower end of the young cirripede, just 

 in advance of its antennae, pressing against the surface of the 

 shell of the mollusc ; and if armed with triturating points, 

 as we have every reason to believe it is, it would wear for 

 itself a cavity. The horny disc on the ventral surface of 

 this protuberant anterior end of the young animal will, we 

 may assume, soon become cemented to the near side of the 

 cavity just supposed to have been excavated. And the whole 

 animal, by further slight changes in direction, namely, by 

 working down more and more obliquely, will take, as shown 

 at (c), its final position. As the whole surface of the animal, 

 with the exception of the horny disc, is provided with tritu- 

 rating points, the animal, when once imbedded, can and does 

 increase its cavity at both ends in length, in depth, and 

 all round the edges of the horny disc, — in short, in every 

 direction excepting directly over the horny disc. I believe, 

 as already explained, that the young Alcippe, (5, diagram), 

 first bores obliquely into the shell ; and whatever amount of 

 downward extension the horny disc attains before the young 

 cirripede assumes its proper position, with its ventral surface 

 upwards and parallel to the inner surface of the shell of the 

 mollusc, that amount determines the thickness of the plate 

 of shell hereafter to be left unabraded over the horny disc, 

 as the latter continues to extend in circumference. This 

 plate of shell over the horny disc is so thin, that, as mentioned 

 at the commencement, the colour of the ovaria is seen 

 through; and until I reflected on the following consi- 

 derations, I was much surprised how the instinct of the 

 animal could so neatly guide it not to grind too deeply, and 

 yet to grind till only a very thin plate of shell was left over 

 its horny disc : these considerations are, that whatever 

 thickness was first given to this plate, of shell, when the 

 animal was very young and first assumed its ultimate posi- 

 tion, that thickness would in most cases be always retained, 

 owing to the flatness of the disc, and to the membrane 

 armed with triturating points protruding very slightly be- 



