394 THE VOYAGE OF H.M.S. CHALLENGER. 
the Crinoid larva have precisely the same relation to the vaso-peritoneal system as the 
corresponding plates (genitals and oculars) of an Urchin is a strong reason for not laying too 
much stress upon the negative evidence of a confessedly imperfect paleontological record.* 
In one respect it is somewhat unfortunate that the Urchins should have been selected 
as affording the typical apical system of the Echinozoa, with which that of a Crinoid 
could be compared. For their apical system is primitively a comparatively simple one ; 
whereas many Crinoids have a ring of plates immediately beneath the basals which are 
unrepresented in the Urchins, though present in many Ophiurids and Asterids. 
In his endeavour to find an early Crinoidal form with a calyx of the same simple 
description as the apical system of an Urchin, Professor Lovén’ was led to select the 
genus Cyathocrinus (Poteriocrinus) ; and he proposed the following homologies between 
the two types :— 
1. Dorsocentral of Urchin =the five under-basals of Cyathovrinus (the basals of J. Miiller). 
2. Genitals of Urchin =the five basals of Cyathocrinus (the parabasals of J. Miiller). 
3. Ocular plates of Urchin = radials of Cyathocrinus. 
The two last of these propositions have been generally, but not universally, accepted. 
As regards the first, however, I am sorry to say that I have found myself unable to 
agree with Professor Lovén. 
I pointed out six years ago® that the under-basals of Cyathocrinus constitute an 
element in the calyx which is by no means so constant in its occurrence as it should be, 
were it a fundamental part of the apical system and homologous with the dorsocentral of 
an Urchin or Starfish. Under-basals are present in Hncrinus, Extracrinus, and Marsu- 
pites among the Neocrinoids, and in Cyathocrinus, Poteriocrinus, Rhodocrinus, and a 
large number of allied genera among the Palocrinoids ; while they are absent in Apio- 
crinus, Pentacrinus, Actinocrinus, Platycrinus, and in many other less known genera. 
When present, there are generally five distinct plates, resting on the upper stem-joint ; 
and this fact, together with the want of constancy in their occurrence, caused me to 
suspect that they could not be collectively homologous to the primitively single dorso- 
central plate of an Urchin or Starfish, as supposed by Lovén. -I was therefore led to 
seek for the homologue of this last in the terminal plate at the end of the stem of the 
Pentacrinoid larva, which occupies the same position with regard to the right peritoneal 
tube as the dorsocentral of a larval Urchin or Starfish. This suggestion has been 
accepted by Liitken and by Sladen, as I have pointed out above (p. 168), though it is 
altogether ignored by Lovén. But no serious arguments have been yet brought forward 
against it by other authors who have discussed the question ; while, on the other hand, 
1 Dr. Hoernes does not appear to have gone into the subject very deeply. I have nowhere suggested that the 
radials of an Urehin are homologous with the basals of a Crinoid; nor that the madreporite of Clypeaster is compar- 
able to the centro-dorsal of Comatula and to the central plate in the calyx of Marsupites. Nevertheless Hoernes thinks 
fit to express his dissent from these views, which have originated with no one but himself, and he entirely misses the 
real point at issue. : 
2 Etudes, loc. cit., p. 80. 3 Quart. Journ. Micr. Sci., 1878, vol. xviii., N. S., pp. 358-361. 
