SUB-GENUS ACASTA. 305 . 



little external points (apparently representing spines), each 

 of which has a short tubulus extending to the corium. 

 Although I dissolved the basis of three specimens, I could 

 not distinctly make out any cement ; nor did I see any 

 cement-ducts ; yet these are readily distinguished, after the 

 dissolution of the basis in acid, in Balanus, Elminius and 

 Tetraclita. There can be no doubt that the young shell 

 must at first be cemented to a fibre of the sponge ; but I 

 suspect that the cementing-tissue is not subsequently formed, 

 owing to the support afforded by the growth of the enve- 

 loping sponge. As some species of Balanus are habitually 

 or occasionally imbedded in sponges, it is important to 

 observe, that the species of Acasta are not only imbedded, 

 but are attached to the fibres of the sponge : but even this 

 character, as we have already seen, is not sufficient to dis- 

 tinguish the genus Acasta from Balanus, for B. declivis is 

 attached exclusively to sponge. 



Perforations in the Shell. — Calcification seems often to 

 fail to a certain extent in this genus : the basal cup in most 

 specimens of A, spongites, and in some of A. glans and 

 other species, is irregularly perforated by numerous minute 

 orifices, closed only by the external membrane, and filled 

 up inside by pulpy corium. In some specimens of 

 A. spongites, from the Cape of Good Hope, parts of the 

 basis were riddled like a sieve. I have seen similar per- 

 forations in the parietes of a few specimens of A. glans. In 

 some specimens of A. sulcata, the radii do not extend quite 

 down to the basal edge of the walls (PL 9, fig. 2 a), and in 

 consequence a small cleft, closed only by membrane, is left 

 between the compartments, for a little space above the basal 

 cup. In A. fenestrata (fig. 7 a), and in a lesser degree in 

 A. purpurata (8 a), not only do the radii not extend to the 

 basal cup, but the parietes either on one or both sides of 

 each suture are hollowed out, so that six, large or small, 

 elongated, membrane-covered openings are formed, which 

 extend from beneath the sheath down to the basal cup. 

 These openings, which I have not seen in any other genus, 

 will be more fully described under the respective species. 



Mouth. — The parts of the mouth are identical in the 

 several species, and present no generic differences from 



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