394 On the Comatulae of the " Challenger " Expedition. [Mar. 6, 



centrodorsal of Act. pectinata with, the openings on the outside of the 

 calyx of Apiocrinus rotundus and Ap. obconicus, which are situated 

 between every pair of continuous basals, and the radials which rest 

 upon them. Other homologues are the radially situated "inter- 

 articular pores " in the upper part of the stem of Pentacrinus. 



It is worth notice, that all the species in which the centrodorsal 

 exhibits these variations of form are true Actinometrce, i.e., they have 

 an eccentric mouth and a terminal comb on the oral pinnules. In 

 Loven's Phanogenia, however, the mouth is central, and there is a 

 terminal comb to the oral pinnules. It is thus a very singular excep- 

 tion, for I know of no Antedon in which the oral pinnules have this 

 terminal comb, nor one in which the centrodorsal has anything like 

 the form which it has in^ Phanogenia. 



In fact, I am able to say that the examination of the " Challenger " 

 Gomatulce has entirely confirmed the opinions held by Dr. Liitken and 

 myself {Actinometra, cap. ii, §§ 14, 15) respecting the distinguishing 

 characters of Antedon and Actinometra. We both agree in referring 

 forms with a (sub) central mouth, five equal ambulacra, and no terminal 

 comb on the oral pinnules, to Antedon. On the other hand, species 

 with an eccentric mouth, a variable number of unequal ambulacra, 

 and a terminal comb to the oral pinnules, belong to Actinometra. 

 There are only two specimens in the " Challenger " collection which 

 have an eccentric mouth but no terminal comb. Pourtales' Comatida 

 meridionalis appears to be another, but these are only three exceptions 

 out of some sixty species. 



It will be seen at once that these characters are of no use in dis- 

 tinguishing the genera of fossil Gomatulce. But, as has been hinted 

 above, there are very considerable differences in the shape of the 

 radials and centrodorsal piece in Antedon and Actinometra respectively, 

 and as these are exactly the parts which are most met with as fossils, 

 the generic determination of a fossil form is almost as easy as that of 

 a recent one, which has given up its disk to produce a Hyponome. As 

 I have described these differences very fully in my Actinometra memoir 

 (cap. iv, § 41, 51, 54-56), it is not necessary to do more than refer 

 to them here, with the remark that a more extended knowledge of the 

 species of both genera has only strengthened the opinions which I have 

 there expressed. 



The same is the case with regard to the so-called " ventral nerve " 

 of Gomatula, viz., the fibrillar band underlying the epithelium of the 

 ambnlacral grooves. I have already shown {Actinometra, cap. iii, 

 § 23-26) that, in Act. polymorpha and Act. Solaris, half, or even more 

 than half, of the arms may have neither groove, epithelium, " nerve," 

 nor tentacles, and I have insisted, as strongly as possible, on the im- 

 portant bearing of this fact on the Ludwig-Gegenbaur view that these 

 subepithelial bands constitute the nervous system of the Crinoids. 



