ON THE HISTOLOGY OF THE PEDICELLARIA. 389 
on the inner surface an apophysis is given off, which serves for the attachment 
of the transverse muscles. 
.. The calcareous rod in the stem is similar to that of the other pedicellarie. 
With regard to the soft parts, the whole pedicellaria is covered with an 
epithelium in no way to be distinguished from that of the ophiocephalous pedi- 
cellariz. The soft part of the stalk between the swollen upper end of the 
calcareous rod and the base of the valves has a similar structure, but the 
looped elastic tissue which forms the upper extremity of the stalk is very feebly 
marked. The transverse muscles arranged in the form of a triangle, and 
attached to the apophysis of the calcareous valves, need no description ; 
everything that has been already said concerning these muscles in the case of 
the ophiocephalous pedicellariz might be repeated here. 
The coiled plates of elastic tissue, three in number, lying inside the 
calcareous valves of the head in the ophiocephalous pedicellariz, we have 
found to exist in the tridactyl pedicellariz, but only after a prolonged search, 
as they are so exceedingly minute and delicate as almost invariably to escape 
observation. Pl. XX., fig. 16, is a transverse section of one of these pedicellarize 
taken just above the base of the head. The specimen has been hardened in 
chromic acid, so that the calcareous parts have been dissolved away, but their 
position can be easily made out, as the apophysis of each lies, of course, 
between each of the muscular masses. The three muscular masses are seen to 
form atriangle. Fig. 17 is a section taken so as to preserve intact the actual 
bases of the calcareous valves. The specimen was preserved in osmic acid. 
This figure may be compared with fig. 14, which is a precisely similar section 
taken from an ophiocephalous pedicellaria; in neither have the delicate 
calcareous valves been at all injured. The valves are covered externally by a 
layer of epithelium, and the three approximator muscles forming a triangle are 
attached to the apophysis of the valves. 
The gemmiform pedicellariz are scattered all over the test, and are about 
equal in number to the last mentioned variety, the ophiocephalous pedicellariz 
being far more abundant than either of these two. Their structure will 
demand a careful and full account, since they differ in many particulars from 
the other kinds of pedicellarie. 
The stalk consists of an internal calcareous rod, resembling the same struc- 
ture in the other pedicellariz, but the knob-like swelling is rather larger. It is 
covered by a layer of epithelium, which is continuous over the whole pedicellaria. 
The soft part of the stalk lying between the calcareous rod and the bases 
of the valves of the head is extremely reduced in size, having, in fact, only 
about half the length of the head. It consists of a bundle of plain muscular 
fibres, in which we have not succeeded in showing striation by the use of 
any reagents. These fibres are attached to the base of the valves, and to 
