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 Bathycrimts,- as in Rhizocrinus, there is "a comparatively excessive 

 development of the vegetative system." This was generally the case throughout the 

 Bourgueticrinidse and Apiocrinidse, 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 Pateocrinoids 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, Pisocrimis, Symhathocrinus, &c. On the other hand, the 

 body and arms, so enormously developed in Croialocrinus, are quite extensive in 

 many Cyathocrinidse and Actinocrinidse ; but the stem is often large and complicated 

 at the same time, as in Barycrinus and Megistocrinus.^ 



In the Liassic Extracrinidaj the stem, immensely developed as it may be, still falls 

 considerably short of the body and arms in the complication of its structure. Extracnnus 

 hriareus 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 " Porciipine " Crinoids, Proc. Hoij. F!nc. Edin., vol. vii. p. 771. 



2 Journ. Linn. Soc. Land. (Zool.), vol. xiii. p. 48. 

 2 Hist. Kat. des Crinoides, pp. 39, 41, 44. 



* The so-called " pinnules " of tlie Blastoids cannot Ije properl)' compared to those of the Crinoids, for they do not 

 seem to have contained the genital glands. 



^ Eevision of the PaL-eocrinoidea, vol. i. pp. 14, 15. 

 <^ Encriniden, pp. 271, 291. 



