366 MR. CHARLES STEWART ON THE SPICULA 
guished, seems to point to a similar mode of development of the skeleton in these 
widely separated members of the animal kingdom. 
With regard to the intimate structure of these spicula, I may mention that many, 
when mounted in balsam, present a much paler appearance than others, probably 
owing to their possessing a less refractive power, and that many apparently identical, 
when examined by polarized light, show a total absence of depolarizing property which 
in others is well marked, and also that the spicular plates of Cidaris frequently have 
their two halves in a state of tension, the one at right angles to the other, although 
no other evidence of structural difference can be detected. 
The proportion of animal matter varies considerably—charring frequently demon- 
strating a line of it running through the centre of the spicula, such as is found in 
those of the Spongiade. 
It would perhaps be interesting to those who may be desirous of extending these 
observations, to describe briefly the means used by me in investigating these structures. 
As all the Echinoidea I examined were in the dry state, and had been for the most 
part preserved for a long time in museums, their softer parts were covered with mould, 
which could only be removed by very careful washing under water with a camel's-hair 
‘pencil: liquor potassæ does not dissolve this fungus; besides, its use would only show 
the separate forms of the spicula; it can, however, be advantageously employed in some 
instances. 
The membranes so cleaned were then dried on a slide, and mounted in balsam in the 
ordinary way. 
As specimens of the genus Cidaris are rare and generally preserved whole, it is 
difficult to procure examples of their internal parts; those of my own cabinet I partially 
sacrificed by removing the anal plates of those in which the oral surface was most perfect, 
and vice versá: by this means the whole of the viscera, &c., could be obtained with- 
out much injury. 
After these general remarks I now proceed to a description of the various modifications 
these spicula present in the different genera I have examined. 
In Cidaris the ovaries, or testes, contain perforated plates, which, in the membranes of 
the base, often attain the 4th of an inch in diameter, and are so crowded as to overlap 
each other. Owing to the rigidity imparted by their presence, the ovarian tubes retail 
their shape in old and dry specimens. Similar plates sometimes extend laterally 02 
each side of the ovarian branches, and in a more delicate form are found in the fibrous 
bands that frequently terminate or connect the branches together (Plate XLVII. figs. l, 
2,3). Their shape is usually irregularly ovate ; but in Cidaris tribuloid , in which the 
ovary is converted into a solid organ by the union of its parts, the plates are evidently 
formed on a triradiate type (Plate XLVII. fig. 5). In another species a portion of the 
plate projected as a spine (Plate XLVII. fig. 4). I have only found one exception to this 
condition of the ovary—in a small Cidaris from Malta, in which all the internal p 
examined were devoid of spicula, although the ambulacral tubes possessed similar ones 
to those of other Cidaride (Plate XLVIII. fig. 12) ; the species probably really belong? | 
to the genus Orthocidaris of Agassiz. 
BE ONE PRE 
