THE ECHINOIDEA 327 



by Echinoids so similar in structure that it is hard to draw a satis- 

 factory line of separation. 



Although the ancestral Echinoid is still unknown, the main 

 lines of evolution in the class are clearly recognisable. The 

 Echinoids began with forms having small, sac-like bodies, and a 

 mouth and anus at opposite poles ; the body was muscular, 

 supported by a series of angular plates, of which five pairs were 

 perforated by pores. At the summit of the" test occurred the 

 apertures of the alimentary, generative, and water - vascular 

 systems ; and the apertures of these systems were supported and 

 held in place by a series of special plates. 



At first the palaeontological record is incomplete, the plates of 

 the test being thin, fragile, and loosely fitted together. Hence 

 there is a gap between the Echinoid just described and its next 

 known successors, in which the interradial plates are irregular, 

 and the apical system of plates is absent. But as the skeleton 

 thickens, fossils become more abundant and better preserved. 

 We can see the increase in the number of interambulacral 

 and of ambulacral plates up to forms such as Melonites. Then, 

 as the plates became stouter, the flexibility of the test was 

 lost ; thus the advantage of having small, numerous plates was 

 lost. Hence the Echinoids with more than twenty rows of plates 

 disappeared, and were succeeded by a group, the main feature of 

 which was the consolidation of the test. About the same time 

 there appeared an offshoot from the main stem, in which the test 

 was strengthened by a great development of the apical system ; 

 this arrangement reached its highest development in two aberrant 

 genera (Tiarechinus and Lysechinus) which lived in the Triassic 

 coral lagoons of the Tyrol. The Melonitoida and Plesiocidaroida 

 apparently left no issue, and all existing and post-Triassic Echinoids 

 appear to have descended from the primitive genera of Cidaroida. 



From Cidaris, with its ambulacra of simple primary plates, the 

 more complex types were developed by the crowding of the pore- 

 pairs, and the decrease in size and increase in number of the 

 spines ; hence the ambulacral plates become compound, and the 

 interambulacral plates bore numerous tubercles and granules, and 

 thus gave rise to the various groups of Regular Echinoidea. In 

 some deep-sea forms the calcification of the external skeleton is 

 imperfect ; the plates are thin and the muscles strong ; by the 

 imbrication and isolation of the plates there is a return to some of 

 the features of the flexible Palaeozoic Echinoids. 



The main departure from the type of regular Echinoids is 

 due to the backward movement of the anus interfering with 

 the originally quinqueradiate arrangement of the organs (Fig. 

 XL VI.). The mouth passes forward, the jaws disappear, the 

 ambulacral podia become specialised for respiration as well as 



