412 BALANID.E. 



attachment is as much owing to the upward growth of the 

 whale's skin, as to the downward growth of the Coronula. 

 In PL 15, fig. 4, a vertical section is given of the whale's 

 skin, taken through the place whence a shell of C. diadema 

 has been removed ; consequently we here see nothing but 

 the whale's skin : the upper black layer is the dark horn- 

 coloured epidermis, forming the general surface of the 

 whale's body, and resting on an orange-coloured fibrous 

 layer, which is lightly shaded in the drawing. The two 

 horns in the section are two of the eighteen projections, 

 formed entirely of the dark epidermic layer, which fill up 

 the eighteen flattened cavities produced by the folding of 

 the walls. Outside the horns we see the section of a circular 

 furrow, in which the circumferential margin of the shell was 

 lodged j and between the horns, there is the central hollow, 

 within which, when lined with shell, the cirripede's body 

 was included. The circular furrow is formed in main part 

 by the epidermic layer being thinner there than on either 

 side, and partly by the orange-coloured, underlying fibrous 

 layer curving a little downwards, from having apparently 

 yielded to the pressure of the circumferential margin of the 

 shell. With respect to the cause of the thinness of the epi- 

 dermis under the circular furrow, I do not know how much 

 to attribute to mere mechanical compression or stretching, 

 and how much to the pressure of the shell, having checked* 

 its formation. In the case of very young and small shells, 

 it is hardly possible that their pressure can have in any way 

 influenced the formation of new epidermic layers under the 

 thick old layers ; and we must believe, at least in these cases, 

 that the whole effect is mechanical, the sharp basal edges of 

 the shell having indented the epidermis; but this is not more 

 surprising than that the radicle of a plant should penetrate 

 hard ground. Whether the indented epidermis in the circular 

 furrow becomes ruptured, I am not sure ; ragged layers may 

 commonly be observed outside the shell, but it is very possible 

 that these may be the ends of layers of epidermis which have 

 been preserved by the covering of the shell, whereas, on the 



* Formerly I was inclined to believe, as stated in my former volume on the 

 Lepadidap, that the cement injured the true skin of the supporting animal, but 

 this, at least in such cases as the present, I do not now at all believe. 



