60 THE VOYAGE OF H.M.S. CHALLENGER. 
enlarged to receive the fertile portions of the genital glands, no part of these appearing 
in the arms (Pl. Ve. figs. 7, 8, 10, ¢; Pl. VI. fig. 1); while they have no appendages of 
their own as the armlets have in Extracrinus. 
The peculiar pinnule arrangement of Hyocrinus helps us to understand why there 
are no pinnules upon the axillaries of multiradiate Crinoids. These may be considered 
as ordinary pinnule-bearing joints, so modified that the pinnule and the continuation of 
the arm which bears it are equal in size or nearly so. As mentioned above, this is in 
fact the mode of formation of the pinnules at the growing points of the arms, as is well 
shown in a very young individual of Pentacrinus decorus (Pl. XXXYV. fig. 1). The 
joint which bears the last formed pinnule is an axillary with two nearly equal distal 
faces; and the pinnule can only be distinguished from the continuation of the arm by 
the greater length of its component joimts. Furthermore, in the short posterior arms of 
Actinometra, the only ones in which the normal mode of termination has been observed,’ 
the last joint is an axillary which bears two pinnules of the ordinary character. 
In Rhizocrinus (Pl. IX. figs. 4, 5) as in Hyocrinus (Pl. VI. figs. 1, 2) the pinnule- 
bearing joints have very much the appearance of axillaries with unequal distal faces; and 
a similar inequality is shown by the axillaries of Hatracrinus, each of which bears an 
“armlet” on one face and the continuation of the main arm-trunk on the other. 
Numerous instances of reparation after injury also indicate the close similarity of 
arms and pinnules. A very common one, sometimes to be met with in Antedon rosacea, 
is as follows :—The epizygal of the third brachial is broken away, carrying with it all the 
outer part of the arm, as well as the pinnule which it bears. But it is replaced by an 
axillary with two distal faces, from each of which an arm eventually grows out, one or 
other of them perhaps dividing again, as in the specimen of Pentacrinus decorus shown 
on Pl. XXXVI. On the other hand, in an abnormal individual of Metacrinus angulatus, 
the eighth distichal is not an axillary, as is usually the case. But it is somewhat swollen 
and has a slightly larger pinnule than the preceding joint, so that it resembles an axillary 
with unequal faces. In the specimen of Actinometra strota which is represented on 
Pl. LV. fig. 2, one of the second brachials of the right posterior ray bears two fully 
developed pinnules instead of an arm and its own proper pinnule, so that it looks like an 
axillary. There is no disk-ambulacrum corresponding to this undeveloped arm. 
Considering therefore the fundamental identity of arms and pinnules, one would 
scarcely expect that an axillary joint which gives rise to two arms (often unequal in size) 
should bear a pinnule as well (see pp. 347, 358). 
The pinnule arrangement of Hyocrinus is totally unlike that of any other Neocrinoid, 
although, according to Sir Wyville Thomson,’ we have something very close to it in some 
species of the Paleozoic genera Poteriocrinus and Cyathocrinus. These names were 
probably employed by Sir Wyville in the wide sense, and not with the restricted meaning 
 Actinometra, loc. cit., p. 40, pl. ii. fig. 6, 2 Journ. Linn. Soc. Lond. (Zool.), vol. xiii., 1876, p. 52. 
