54 ‘ THE VOYAGE OF H.M.S. CHALLENGER. 
united to the preceding joint by a bifascial articulation instead of by syzygy. In fact it 
is a general rule in all Crinoids that pinnules are only borne by those joints which are 
united to their successors by paired muscular bundles. 
The hypozygal in the brachial syzygies may be fairly considered as losing its indi- 
viduality. Not only does it bear no pinnule, but it takes no part in the movements of 
the arm. But when two joints are united by ligamentous bundles on either side of a 
vertical ridge, they are able to share in the lateral movements of the arm, though not in 
those of flexion and extension ; and it therefore seems unreasonable to consider a pair so 
united as equivalent to one joint only. 
Sir Wyville Thomson was accustomed to regard the stem and its appendages as 
constituting the “vegetative system” of the Crinoid, as distinguished from the more 
strictly animal portions, viz., the cup and arms. In describing Rhizocrinus for example,’ 
he specially alluded to the great ‘“ preponderance in bulk of the vegetative over the more 
specially animal parts of the organism ;” and he subsequently pointed out that in 
Hyocrinus and Bathycrinus,? as in Rhizocrinus, there is ‘a comparatively excessive 
development of the vegetative system.” This was generally the case throughout the 
Bourgueticrinidee and Apiocrinide, none of which have any very great number of arm- 
joints, though the “body” may be considerably enlarged with the help of the upper 
part of the stem. Thus, for example, d’Orbigny’® describes two twenty-armed species 
of Millericrinus, each reaching a total length of one metre, out of which the calyx and 
arms together only take up 86 and 94 millimetres respectively, less than one-tenth of 
the whole; while in one ten-armed species the calyx and arms together only measure 
29 out of 920 millimetres. 
Among the Palocrinoids there is considerable variation in the relative development 
of the stem as compared with the body and arms. The latter are often absent altogether, 
as in the Blastoids* and many Cystids; while they are few in number and poorly 
developed in Haplocrinus, Pisocrinus, Symbathocrinus, &e. On the other hand, the 
body and arms, so enormously developed in Crotalocrinus, are quite extensive in 
many Cyathocrinide and Actinocrinide ; but the stem is often large and complicated 
at the same time, as in Barycrinus and Megistocrinus.’ 
In the Liassic Extracrinidee the stem, immensely developed as it may be, still falls 
considerably short of the body and arms in the complication of its structure. Extracrinus 
briareus has a comparatively short stem; but in Extracrinus subangularis it may 
exceed 50 or even 70 feet,° with but few cirri except near the calyx, and those 
1 “Porcupine” Crinoids, Proc. Roy. Soc. Edin., vol. vii. p. 771. 
2 Journ. Linn. Soc. Lond. (Zool.), vol. xiii. p. 48. 
3 Hist. Nat. des Crinoides, pp. 39, 41, 44. 
4 The so-called “pinnules” of the Blastoids cannot be properly compared to those of the Crinoids, for they do not 
seem to have contained the genital glands. 
5 Revision of the Palzocrinoidea, vol. i. pp. 14, 15. 
§ Eneriniden, pp. 271, 291. 
