236 PHYLUM ECHINODERMATA. 



in this interradius or in the next radius (radius No. I of our 

 enumeration, Fig. 83). In the irregular Echinoids the anus 

 lies in the next interradius but one (interradius No. V. I of 

 our enumeration, Fig. 83) the so-called posterior interradius 

 of the Exocyclica, and the question arises which position is most 

 primitive, the subapical position in interradius I. II, or in radius 

 I of the Endocyclica, or the position in the Exocyclica in which 

 it lies remote from the apex in interradius V. I. It is impossible 

 to answer this question. On the one hand the similarity between 

 the Asteroids and regular Echinoids on this point suggests that 

 the subapical position is primitive, but this similarity may be 

 apparent only. On the other hand there is plausibility in 

 Loven's contention that its position in the posterior interradius, 

 recalling as it does the condition found in Palaeozoic Crinoids, is 

 really primitive. Both these arguments, however, rest upon 

 an unproved assumption, viz. that the homologies between the 

 different radii and interradii of the classes of Echinodermata 

 have been determined. We know nothing on this subject, but it 

 must be admitted that, taking into account the obvious affinities 

 between the Asteroids and Echinoids, there is more probability 

 in favour of the homologies implied by the first view than in 

 those implied by the second ; for the Crinoids are the most out- 

 lying group of living Echinoderms. 



The development of Echinoidea, though differing in many 

 points from that of Asteroids, bears out on the whole the view 

 as to their affinities suggested by their adult structure.* 



On account of their marine habits and the structure of their 

 body-wall the Echinoidea are well suited for preservation as 

 fossils, and an immense number of extinct species are known. 

 Writing in 1881 A. Agassiz estimated that 2,000 fossil and 225 

 recent species were known and the number is now probably much 

 greater. The study of these forms and the elucidation of their 

 affinities present problems of the greatest importance to the 

 student of organic evolution. Of the extant families the Cida- 

 ridae alone are found in Palaeozoic formations. The Diadema- 

 toida, Holectypoida, Cassidulidae and Collyritidae begin in the 

 Jurassic. The Spatangidae are not found till the Cretaceous, and 

 the Clypeastridae do not make their appearance before the upper 



* See MacBride, Phil. Trans., 195, 1903, p. 316. 



