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 palseontological record.^ 



In one respect it is somewhat unfortunate that the Urchins should have been selected 

 as affording the tj^ical apical system of the Echinozoa, wdth 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 Loven* was led to select the 

 genus Cyathocrinus (Poteriocrinus) ; and he proposed the following homologies between 

 the two types : — 



L Dorsocentral of Urchin = tlie five under-basals of Cijathocrinus (the basals of J. Miiller). 



2. Genitals of Urchin = the five basals of Cyatliocrimis (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 Loven. 



1 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 Encrinus, Extracrinus, and Marsu- 

 pites among the Neocrinoids, and in Cyathocrinus, Poteriocrinus, Rhodocrinus, and a 

 large number of allied genera among the Palteocrinoids ; while they are absent in Apio- 

 crinus, Pentacrinus, Actinocriniis, 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 Loven. 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 j)osition wath 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 Loven. But no serious arguments have been yet brought forward 

 against it by other authors who have discussed the question ; w^hUe, on the other hand, 



' Dr. Hoenies does not appear to have gone into the sulyect very deeply. I have nowhere suggested that the 

 radials of an Urchin are homologous with the basals of a Crinoid ; nor that the madrejiorite of Clypeaster is compar- 

 able to the centro-dorsal of Comatula and to the central plate in the ealj'x of Manupites. 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. at, p. 80. 3 Quart. Journ. Mkr. Sd., 1878, vol. xviii., N. S., pp. 358-361. 



