171 



FAMILY IV.— ACTINIAD^E. 



The species of this family, though very tew in number, 

 are well marked by the single character of being furnished 

 with those peculiar organs which M. Milne-Edwards call? 

 (not very felicitously) bourses chromatophores, or tubercuhs 

 calicinaux, and which I have named marginal spherules. 

 These are hollow spherical vesicles, with thin walls, situ- 

 ated near the edge of the disk, on the inner side of a sharp 

 margin, and outside the exterior row of tentacles. For the 

 most part, if not always, these organs are of bright or 

 vivid colours, generally differing from those of the other 

 parts ; and hence they are conspicuous, and impart a 

 peculiar aspect to the physiognomy. 



What function in the economy of the animal is per- 

 formed by these bead-like spherules is as yet unknown, 

 though that they play some important part can scarcely be 

 doubted. In our Actinia mesembryanthemum, I have ascer- 

 tained that the walls are almost wholly composed of cnia\t\ 

 of nearly linear form, and about *0025 inch in length. The 

 inclosed thread is with difficulty seen, both before and after 

 extrusion ; it is, however, of considerable length. From this 

 structure I have conjectured that the marginal spherules 

 in this family may represent — functionally, not homo- 

 logically — the acontia of the Sagartiadce, which are here 

 wanting. 



Sir John Dalyell has an extraordinary observation to 

 the effect that each of these spherules " is pierced by 



