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



above. Further, these plates "are large and well-developed in the adult forms in 

 Pentagonaster semilunatus, Gymnasteria carinifera, in various species of Pentaceros, 

 and a large number of the Goniasteridee." 



It thus appears that a small nuniber of Ophiurids and a larger number of Starfishes 

 have two rings of plates between the radials and the dorsocentral, just as is the case in 

 Marsupitcs ; though in the majority of the members of both groups there is only one 

 ring, the elements of which are interradial, as is the case in all the Urchins, so far as 

 is yet known. The homology of these interradial plates with the basals of the Penta- 

 crinoid larva and of Marsupites is a part of Loven's theory ; while he long ago pointed 

 out the homology between the central abactinal plate of the Starfish larva and the dorso- 

 central of Marsupites} I do not see therefore, how he can do otherwise than accept 

 the views of Sladen and myself respecting the homology of the radial plates immediately 

 surrounding the dorsocentral of Ophiomusium, Amphmra, Asterias, and Zoroaster with 

 the under-basals of Marsupites. In both cases these radial plates separate the dorso- 

 central from an interradial series which are called basals in a Crinoid and genitals in 

 an Urchin or Starfish, i.e., the plates for which Loven proposes the general name of 

 "costals." I do not imagine that he will deny (1) that the radial plates between the 

 costals and dorsocentral of Marsupites are homologous with the radial plates between 

 the costals and dorsocentral of the young Asterias; nor (2) that these radial plates are 

 homologous with the under-basals of Cyathocrinus, which are immediately within the 

 basals, or as he prefers to call them, costals. But, according to his present view, these 

 under-basals of Cyathocrinus represent the dorsocentral of the young Asterias. There 

 is, however, no reason to seek for the homologue of the five under-basals of a dicyclic 

 Crinoid in the single dorsocentral of a larval Starfish ; for this dorsocentral is 

 surrounded by five plates which correspond exactly in their relative positions with 

 the under-basals of Marsiqntes, and therefore of other dicyclic Crinoids, including 

 Cyathocrimis. 



If I may be permitted to use here an expression employed by Prof. Loven with respect 

 to another homology which he establishes, I would say that " to anyone believing in the 

 consistency of Nature's ways, there is no reason whatever for doubting " that the apical 

 systems of some Asterids and Ophiurids contain plates which are truly homologous with the 

 under-basals of a Crinoid. It is unfortunate that their presence was not discovered in 

 time to be noticed by Prof. Loven in his recent discussion of the question; for I am 

 sanguine enough to believe that it would have led him to reconsider his views respecting 

 the homology of the five under-basals of a Crinoid with the primitively single dorso- 

 central of a Starfish. 



The interradial plates in the apical system of a Starfish or Urchin are usually known 

 as the genitals; but this term, " besides being expressive of incidental relations peculiar 



' Etudes, loc. cit., pp. 72, 86. 



