Prof. S. Loven 07i the Structure of the Echinoidea. 379 



entire ; then they arrange themselves in groups of three, two 

 half aboral and an entire adoral. Gradually the upper aboral 

 plate increases and becomes entire, and, together with the 

 adoral plate, embraces the intermediate half plate. But these 

 groups do not unite into large plates in the neighbourhood of 

 the peristome by the coalescence of the sutures as in the 

 Latistellge. Echinoneus again agrees with the latter in that 

 the alteration of the plates, which is a consequence of their 

 growth during the accession of new ones from above, takes 

 place in a uniform manner throughout the whole ambulacrum, 

 which, moreover, throughout, from the peristome to the vertex, 

 bears no other than cylindrical tentacles witli sucking-disks, in 

 which there is a ring of calcareous network. This is not the 

 case in the other three groups of irregular Echinoidea, in which 

 the branchia3 have a portion (in the Cassidulidas and Clypeas- 

 tridte in all the five ambulacra, in the Spatangidse in most 

 cases in the four paired ones) — a portion which during 

 growth acquires the leaf-like form to which the name of 

 petalum is given, whilst the lateral plates nearly retain their 

 first form, and the buccal area, with its peculiar tentacles, is 

 gradually compressed and altered. While in the Echinidte 

 and Clypeastridae the peristome in its firm union with the 

 masticatory apparatus continues circular or five-cornered as it 

 was from the beginning, although in some {e. g. Echinometra 

 or Echinocidaris) it deviates therefrom with age in some 

 degree, its character in the Cassidulidge and Spatangidee, 

 which do not possess a masticatory apparatus, is quite dif- 

 ferent. In both it alters its form during growth ; how it may 

 be in Echinoneus remains to be ascertained. When a Cassi- 

 dulus is still quite young, the peristome is pentagonal, with 

 rounded angles, and the ambulacra occupy larger portions of 

 its margin than the interradia, with the exception of the 

 labrum ; in full-grown examples the conditions are reversed, 

 inasmuch as the interradial peristomial plates, especially 

 in 2 and 3, become swelled up during growth, and give 

 the pentagon the incurved sides which are characteristic of 

 this group, and between which the first plates of the ambulacra, 

 compressed into a wedge-shape, enclose the projecting angles. 

 But the mouth, gradually elongated transversely, remains in 

 the middle of its naked membrane. Connected with this trans- 

 formation of the peristome are the compression and displace- 

 ment of the primary plates situated near the peristome (which 

 take place with age), and that considerable alteration of their 

 original relations by which that arrangement is produced 

 which Desor calls jphyllode. We have still to investigate the 

 origin and progress of this in individuals of diiferent ages. 



27* 



