12 THE VOYAGE OF H.M.S. CHALLENGER. 



II.— THE STEM AND ITS APPENDAGES. 



We have already seen that three types of stem occur in the recent Crinoids, which 

 are characteristic respectively of the families Pentacrinidas, Boui-gueticrinidse, and 

 Hyocrinidfe. Broadly speaking, it may be said that these embrace all the varieties of 

 stem which are met with in the Neocrinoidea. For the mode of union of the joints in 

 Apiocrinus, Millencrinus, and Eugeniacrinus must have been very much what it is in 

 Hyocrhms ; though the " root " of the first-named genus is somewhat peculiar in its 

 nature. 



A. PENTAOPaNID^. 



In all the genera of this family the stem consists of discoidal joints which are never 

 (in the adult) higher than wide, and have the characteristic petaloid markings on their 

 terminal faces (Pis. XL, XIV.; Pis. XV. figs. 4, 5 ; Pis. XIX., XXII ; PI. XXVI. 

 figs. 12-18; Pis. XXVIIL, XXIX; PL XXX. figs. 25-30; PL XXXa. figs. 1-7; 

 PL XXXVn. figs. 10-22; PL XXXIX. figs. 3-11; PL XLL figs. 1-5, 6-8, 15-17; 

 PL XLVII. figs. 1-9). Certain of these joints, separated from each other by intervals of 

 variable length, bear whorls of cirri, and they have consequently received from Sir 

 Wyville Thomson^ the very appropriate name of " nodal joints." There ai-e usually five 

 cirri at each node, situated in the direction of the rays (Pis. XL, XVIII. , XIX., XXXIV.- 

 XXXVL, XL., XLII.-XLVL, XLVIII.-LIL). Sometimes, however, one cirrus is 

 deficient, as shown in the left hand figure on PL XXXVI. ; while in Pentacrinus 

 alternicirrus there are only three cirri at one node, and two at each of those above and 

 below it, the positions of these two corresponding to the two gaps at the node of three 

 cirri (PL XXV.; PL XXVI. figs. 13, 14; PI XXVII. figs. 1-3). The cirri of two 

 successive nodes therefore alternate in position as the leaves do in the stem of a Labiate 

 plant, the two faces of the stem which bear no cirri at one node being the only ones which 

 have cirri at the nodes above and below it. 



The function of the cirri, which vary considerably both in length and in stoutness, is 

 described by A. Agassiz ^ as follows : — " These they move more rapidly than the arms, and 

 use them as hooks to catch hold of neighbouring objects, and on account of their sharp 



' Sea Lilies, p. V. 



- Letter No. 3, on the dredging operations of tlie U. S. Coast Survey, Sr. " Blake," from December 1878 to March 

 10, 1879, Bull. Mus. Comp. Zoijl. vol. v. No. 14, p. 296. 



