CLASS I ECHINOIDEA 269 



to those of Ophiurans and Starfishes, but have little in common with the 

 larvae of Crinoids. The later stages in development are of great morpho- 

 logical and phylogenetic interest. Stages in development can, of course, be 

 best obtained by studying young individuals, but, as shown by Jackson, 

 they can also in a measure be obtained from a study of adults. The plates 

 at the ventral portion of the test are the oldest and first formed, excepting 

 as some may have been resorbed in the advance of the peristome. Ventrally, 

 therefore, stages in development are often observable in both the ambulacra 

 and interambulacra, this condition being especially marked in the Perischo- 

 echinoida. Dorsally are found the youngest plates of the test, and it is here 

 that we observe localised stages in development. For, as we pass ventrally 

 to the progressively older plates, it is found that characters are taken on 

 in regulär sequence which present stages directly comparable to those seen 

 in the youthful development of the individual. Dorsal localised stages are 

 especially marked in the ambulacra of those types where a complex structure 

 is attained, as in the Palaeechinidae and the Centrechinoida. The apical disk, 

 periproct, peristome, lantern and perignathic girdle all show stages in 

 development with suitable material, which stages are directly comparable 

 to adult conditions of simpler members of the group, and therefore are of 

 great phylogenetic significance. 



Among Echinoids, as elsewhere in invertebrate groups, evidence is accumu- 

 lating that evolutionary Variation is not radial in all directions, but rather is 

 in definite directions, or orthogenetic. It would appear that the majority of 

 variations are either arrested, in which cases the variant retains characters 

 displayed in its own youthful stages and typical of the adults of more primi- 

 tive allies ; or progressive, when the variant has characters not typical of the 

 species, but which are further evolved in the direct line of difFerential develop- 

 ment. These latter are seen typically in more highly evolved closely related 

 species or genera. In order to study Variation intelligently it is of prime 

 importance to be familiär not only with the characters of the associated 

 species and genera when considering any given case, but also with the develop- 

 mental characters of the same. Variation needs to be especially considered in 

 undertaking phylogenetic studies. 



Homologies. — The Echinoidea differ radically from the Pelmatozoa and 

 Asterozoa in that arms are completely wanting. They differ from Crinoids in 

 that reproductive glands are within the test and interradial, that ambulacral 

 and interambulacral plates originate on the ventral border of a fixed plate, the 

 ocular, and in the possession of a lantern. Echinoids differ from Starfishes 

 essentially in that radial water, nerve and blood canals are on the proximal not 

 distal side of the ambulacral plates ; that ambulacral pores pass through, not 

 between the plates ; and in the possession of a lantern. 



Habits. — Echini are exclusively marine, and are more or less gregarious. 

 Many species occur in littoral zones, and from that region various species and 

 genera extend to continental and abyssal depths. Echini commonly live on 

 the surface of the sea floor, or cling to rocks. Some Echini burrow in sand, 

 others (Strongylocentrotus, Echinometra) along the coast occur in cavities which 

 they bore in solid rocks. The same species does not excavate in sheltered 

 places. 



- About 500 recent species are known, as compared with fully 2500 fossil. 

 The earliest types appear in Ordovician rocks (Bothriocidaris), and continue to 



