STUDIES IN SPICULE FORMATION, 551 
Plymouth two living fairly adult specimens of another 
common genus of the Cucumariide—Thyone. ‘These I 
prepared for microscopic examination of the spicules by the 
method usually employed by me (vide supra), and I am thus 
enabled to describe the scleroblastic mode of development of 
some of the spicules characteristic of this genus. Needless 
to say, this mode of development is essentially identical with 
that described for the Cucumarian spicules. In preparing 
Thyone it is very difficult, if not impossible, to fix the animal 
with osmic acid in a fully-distended condition, and one has 
to be content (unless, perhaps, anesthetics are employed) 
with those few portions of the body-wall which happen to be 
so fixed, since the remainder is of little use. In Cucumaria 
the multitude of spicules present in the body-wall prevents 
this undue contraction. The muscle-layers have, as before, 
to be carefully removed before mounting the portion of 
integument to be examined. 
In Thyone fusus—the species obtained—there exist five 
distinct classes of spicular elements, which may be briefly 
enumerated as follows :—(a) the circular perforated-plate or 
“wheel” spicules situated at the extremities of the podia 
(text-fig. 2); (b) the “stool” spicules (figs. 50—56), chiefly, 
but not exclusively, found in the region of the integument 
situated just posterior to the ring of tentacles, and which is 
devoid of podia, the plates of which spicules are also circular 
in outline, but larger meshed centrally and thicker in limb as 
compared with the podia wheels, also they are smaller in total 
circumference (cf. text-figs. 2 and 3); (c) the much smaller 
irregular superficial spicules (figs. 57—65) found in the same 
region of the bedy as those last mentioned, more particularly 
at the bases of the tentacles; (d) the spicular “ sponge-work”’ 
composing the oral plates, and (e) very large, massive, 
approximately-circular, perforated plates (three or four times 
the size of the podia wheels) situated in the main body-wall. 
These last I only discovered in isolating the spicules from the 
softer parts of the animal by means of eau-de-javelle, and I 
am ignorant as to their exact situation in the body-wall. My 
