56 THE VOYAGE OF H.M.S. CHALLENGEE. 



the Pentacrinidse show very clearly that they are rightly regarded as permanent larval 

 forms of the Comatulas. 



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 

 (PL VII. figs. 2, 7 ; PI. VIII. figs. 1,3, 5 ; PI. Villa, 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 Comatulse ; ^ and in one genus, Atele- 

 crinus, the first pinnule is as far out as the twelfth brachial, the lower pinnules not 

 developing at all. In Rhizocrinus raivsoni 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 (PL 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 VIII. figs. 1, 3). 



In 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. 

 fio-s. 1,2; PL LVI. figs. 7, 8) ; and there is frequently a considerable amount of difi"erence 

 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 ofi" 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 difi"erent. 



A great many species of Actinometra, however, are characterised by a still more 

 striking difi"erence 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 pinnules 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. 



* Preliminary Report on the ComatulcC of the Caribbean Sea, Bull. Mus. Comp. Zool., vol. ix., No. 4, pp. 14, 15. 



