56 THE VOYAGE OF H.M.S. CHALLENGER. 
the Pentacrinidee show very clearly that they are rightly regarded as permanent larval 
forms of the Comatulee. 
Rhizocrinus and Bathycrinus, with their relatively large vegetative system, manifest 
the same character in another way, viz., the absence of pinnules from the arm-bases; 
though the ambulacral plating is continued to the end of both arms and pinnules 
(EVIL fas: 2.75 Pl. VIlEsiee ies. SeePe VUla. fig. 1). There is good reason to 
believe that the late appearance of the basal pinnules (excepting on the second brachial) 
is a marked developmental character among the Comatule;* and in one genus, Aftele- 
erinus, the first pinnule is as far out as the twelfth brachial, the lower pinnules not 
developing at all. In Rhizocrinus rawsoni it is on the epizygal of the third syzygial pair, 
or the sixth primitive joint, and in Rhizocrinus lofotensis on the epizygal of the fourth 
pair (PI. IX. figs. 1-3); while in Bathycrinus it may be as many as eleven joints from 
the radial axillary, though occasionally only eight or nine (Pl. VIIL. figs. 1, 3). 
Tn all the genera of living Crinoids, with one singular exception, the mouth is situated 
at or near the centre of the disk (Pl. III. fig. 2; Pl. VI. fig. 4; Pl. VII. fig. 3; Pl. XVII. 
figs. 6,10; Pl. XXVI. figs. 1,2; Pl. XXXIV. fig. 2; Pl. XXXIX. fig. 2; Pl. LV. figs. 3-7 ; 
Pl. LVI. fig. 6), and the arms are about equally developed on all the five rays. But in 
the large Comatulid genus Actinometra the mouth is excentric or even marginal (Pl. LV. 
figs. 1, 2; Pl. LVI. figs. 7,8); and there is frequently a considerable amount of difference 
in the development of the oral or anterior, and the aboral or posterior arms. 
Even when all the arms are provided with food-grooves on the ventral surface as in 
other Crinoids, those which come off round the mouth are usually longer, sometimes 
considerably so, than those which spring from the hinder part of the disk; while in other 
species the anterior and posterior arms are all grooved and all equal in length, but the 
distribution of their syzygies is quite different. 
A great many species of Actinometra, however, are characterised by a still more 
striking difference between the anterior and posterior arms. The former have a wide 
food-groove of the usual character and a well-developed tentacular apparatus at its sides, 
while they always appear to end in a “ growing point.” The posterior arms, on the other 
hand, have an ungrooved and convex ventral surface, which is without any respiratory 
tentacles at all (Pl. LVI. fig. 7). They are only about half the length of the grooved 
anterior arms, and, therefore, taper much more rapidly, while they terminate in an 
axillary segment which bears two pinnules of the ordinary character. The genital glands 
which they contain are usually far more developed than those of the anterior arms. Not 
only are there more fertile pinnules, though the total number of pimnules may not be 
much more than half that of an anterior arm; but the portions of the glands within these 
pinnules also attain a greater size than in the oral arms, the basal and median pinnules 
of the latter being usually less swollen than the corresponding pinnules of a posterior arm. 
1 Preliminary Report on the Comatule of the Caribbean Sea, Bull. Mus. Comp. Zoél., vol. ix., No. 4, pp. 14, 15. 
