REPORT ON THE CRINOIDEA. 83 
In many of the tropical Comatul the pinnule-ambulacra are fully as well or even 
better plated than those of the Pentacrinide. Antedon acoela and Antedon inequalis,' 
both from Stations where Pentacrinidze are abundant, are good instances of this (Pl. LIV. 
figs. 4, 6-9). The differentiation of side plates and covering plates is more complete ; and 
the plates themselves are not only larger relatively to the pinnule-joints, but also absolutely 
so. When the covering plates are erected and the groove opened, as shown in Antedon 
inequalis (Pl. LIV. fig. 8), the tentacles are extended between them. But the tentacles 
can be completely retracted and the plates closed down so as to convert the grooves into 
tunnels, as shown in Antedon acoela (PI. LIV. fig. 4). 
In the distal edge of each of the side plates is a small rounded notch, so that there 
is a series of gaps along each side of the ambulacrum, one between every two plates. 
These lodge the problematical “ sacculi” which are so characteristic of the genus Antedon. 
In those Comatule with plated pinnules which have short and rounded genital 
glands, instead of the long fusiform structures characteristic of Antedon eschrichti, Hyo- 
erinus, and the Bourgueticrinide, there is sometimes a curious modification of the 
perisomatic skeleton on the genital pinnules. The enlarged part of the pinnule is 
protected by a very strongly developed anambulacral plating, which is much more 
regular and closely set than that of the disk and arm-bases, especially in the case of 
Antedon acoela (Pl. LIV. figs. 1-3). Resting upon the four or five middle joints of the 
short pinnules there is a double row of large plates, which are rectangular at the base 
but somewhat more irregular in shape at their upper ends. There are generally five or 
six plates in each row; but those of the two sides have no fixed relative positions, 
sometimes corresponding exactly, and sometimes alternating as exactly. They have 
the same protective function and very much the same appearance as the large side plates 
of Hyocrinus (Pl. Ve. figs. 9, 10, sp), but differ from them in two ways. In other 
species of Antedon, such as Antedon angusticalyx and Antedon incerta (BE ALLY, 
figs. 5, 6), these protecting plates are smaller and more irregular than in Antedon acoela; 
while in Antedon incerta the two rows are separated by the ambulacrum with its well 
developed side and covering plates. But im Antedon angusticalyx and in Antedon acoela 
these swollen lower pinnules receive no branches from the brachial food-groove, Just as 11 
many species of Actinometra; and the anambulacral plates covering the genital glanas 
consequently meet one another in the medio-ventral line of the pinnule above the gland 
within. The sacculi, however, which he at the sides of the ambulacra may extend on 
to these grooveless pinnules, and occupy small holes between the large protecting plates; 
while in the outer joints of the pinnules, beyond the glands, the sacculi occupy the 
median groove on the upper surface of the skeleton, as is well shown in Antedon angusti- 
calya (Pl. LIV. fig. 5). 
1 The specific formula of this typeis— A. 8(1. or 2p.) e : 
