ON THE COMMON BRITTLE STAR. 47 
I am fully satisfied from a most careful examination of a large 
number of spines, in all stages of growth. 
The peculiar circumstance of one side of the spine having much: 
larger ‘‘teeth” than the other, is also worthy of attention; and 
it would appear that the side passing the larger teeth is that 
which in the early or hooked stage possessed the fangs, as shown 
at Fig. 6. 
The subject of the growth of these calcareous structures is one 
of great interest, and when the various parts of the Echinodermata 
are microscopically examined, our admiration and wonder are 
excited by observing the singular modification which they undergo, 
many of them very difficult to understand. As from the extremely 
small proportion of fleshy matter that these structures contain, 
we are led to expect, that the solid or calcareous portion when once 
formed, must henceforth retain (within certain limits) the shape 
and distinctive features of the original mould. But that this is 
not the case, the foregoing will already show, and I doubt not 
there are many other phases in the growth of the organs and parts 
of this interesting class that would well repay careful study.* 
The gradual development of the plates on the disc presents 
some interesting features difficult to understand from the great 
variety of forms met with, no two specimens fully agreeing with 
each other in the shape of the plates clothing the disc. The 
general principle and features may however thus be stated. The 
central surface of the disc in early stages is furnished with a 
transparent membrane of a calcareous nature, at first thin and 
devoid of perforations, which however soon appear, and are gene- 
rally arranged in rows, forming between them five divisions. The 
central plate or membrane then thickens and breaks up into six 
scales, five being ranged round the central portion, the lines of 
fracture having been determined by the small perforations. The 
last formed scales now rapidly assume the features of those on 
the outer margin, which in all probability were also formed 
* IT may here remark that the possession of hooks in an early stage, to be afterwards 
transformed into spines, is not confined to the species under consideration: a young speci- 
men of the Daisy Brittle Star (Ophiocoma bellis) in my possession, shows the hooked 
forms in all stages, in a marked and most interesting manner ; and itscems highly probable 
that other species of similar character will be much the same. 
