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FAMILY IV.— ACTINIAD^. 
The species of this family, though very few in number, 
are well marked by the single character of being furnished 
with those peculiar organs which M. ^lilne-Edwards calls 
(not very felicitously) bourses cliromatoi^Jiores, or tuherculcs 
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 colom-s, 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 mesenibryanthemum, I have ascer- 
tained that the walls are almost wholly composed of cnidce, 
of nearly linear form, and about *0025 inch in length. The 
inclosed thread is with difficulty seen, both before and after 
extmsion ; 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 
