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XII.. On the Spicula of the Regular Echinoidea. By CHARLES STEWART, Esq. 
Communicated by Professor HuxLey, F.L.S. 
(Plates XLVIL, XLVIIL, XLIX., L.) 
Read November 16th, 1865. 
THE subject of the present paper is an account of those caleareous bodies that are 
found imbedded in certain parts of the perisoma of the Echinoidea, and also in the 
membrane and its reflections that line the interior of their shells. They are generally 
. to be found in the external branchiæ and ambulacral tubes, and internally in the 
madreporic canal, ovaries, intestine, and mesentery, and sometimes in the ambulacral 
vessel and the membranes and ligaments about the base of the jaws. 
Although the forms of the spicula of the Cirrho-vermigrade Echinodermata have been 
found to be of the greatest value in the determination of species, but little notice has 
been taken of them in the regular Echinoidea. | 
Valentin, in his monograph on the genus Echinus, mentions their occurrence in 
various parts, and gives M. Miescher as an authority for their being found in the ovary, 
but strangely remarks that “it remains to be proved whether these little calcareous 
bodies are artificial products, or exist as well in the living animal." He figures those of 
the external branchiæ of Echinus lividus, and of one called by him Æ. brevispinosus, 
Which, however, I believe is identical with E. Dröbachiensis of our northern coasts, the 
spicula, which are very peculiar, being identical with those of that species; and I have 
since been confirmed in this opinion by Dr. Herapath, of Bristol, whose examination 
of the pedicellaria led him to a similar conclusion. 
These spicula vary greatly in the amount of their development in different genera 
and species; and in this respect, as well as in their different forms, they will, I believe, 
be found to afford most valuable and interesting additional points of generic and specific 
distinction. 
The bihamate is the usual shape they present. Such are figured by Valentin, mixed 
With reticulated plates as they occur in the external branchiæ ; to these must be added 
perforated or reticulated plates often attaining a large size, having irregular or smooth 
margins, and triradiate, acerate, biclavate, bihamate, and irregularly branched or curved 
Spicula. In many, more especially in those of the bihamate form, there is a great ten- 
dency to the formation of a spheroidal enlargement at the centre, which sometimes, by 
increased development, forms a spine projecting either from their concave or convex 
border ; many specimens seem to show that this spheroidal body is often first formed, 
and that from it the other parts of the spiculum are subsequently extended. Between 
all these forms intermediate conditions can be found, by which their identity in struc- 
ture and mode of development with the other shelly parts can be clearly demonstrated. 
The Striking resemblance to the spicula of Sponges and to those of some Mollusca, 
from. which many, particularly those of Echinometra, could not, I think, be distin- 
a VOL. Xxv, 3 F 
y/ 
