EVOLUTION OF THE ECHESTODERMS — FELL 459 



Nonetheless, the vast majority of echinoderms do exhibit radial sym- 

 metry, and it is the radially symmetrical members that I wish to 

 discuss here. 



Figure 1 sets out the major divisions of the phylum, arranged in ac- 

 cordance with the conventional classification. On the left are the 

 crinoids, which, with certain other fossil groups, comprise the sub- 

 phylum Pelmatozoa. These forms are attached to the substrate for at 

 least the earlier phases of the life history; they have a U-shaped gut, 

 so that the mouth and anus lie side by side on the upper surface, and 

 they employ a ciliary feeding mechanism which sweeps small particles 

 along radial food grooves to the central mouth. The other four 

 groups of echinoderms form the subphykmi Eleutherozoa, f ree-livmg 

 animals not attached to the substrate, with either a blind gut or a 

 straight gut, the mouth being on the lower side, and equipped with 

 jaws or similar structures, permitting a much coarser mode of nutri- 

 tion. They comprise the Holothuroidea or sea cucumbers, the 

 Asteroidea or starfishes, the Ophiuroidea or brittlestars, and the 

 Echinoidea or sea urchins. All Eleutherozoa are viewed as descended 

 from some common, though unlmown, ancestry. This classification 

 has been in vogue for many years, but there are grounds for suspecting 

 that it is incorrect. However, before setting out on a necessarily 

 abbreviated review of the evidence, we may first take note of certain 

 views introduced by embryologists earlier in this century. 



LARVAL FORMS 



Figure 2 shows in schematic form a phylogenetic arrangement of 

 Eleutherozoa as suggested by a comparison of their larval forms. 

 Sea urchins and brittlestars share a larva called the pluteus, as seen 

 in the upper two rows. Strictly speaking the larvae of the two groups 

 are not the same, but they are certainly vei^- similar and distinguish- 

 able from all others by their paired, rodlike arms. On the other hand, 

 sea cucumbers and starfishes have larvae which lack rodlike arms, 

 barrel-shaped forms with ciliated bands or lobes ; in the simplest form, 

 such a larva is called an auricularia. It is shown in the two lower 

 rows. On the right you see the adult forms into which they meta- 

 morphose, and on the left the earliest larval stage of all, the di- 

 pleurula, from which all the larvae can be derived. Crinoids have 

 a different type of larva, not shown in this diagram, but which we 

 will discuss briefly a little later on. 



The following phylogenetic relationships have been inferred from 

 such evidence: Ophiuroids and echinoids are supposed to descend 

 from some common ancestor while asteroids and holotliurians would 

 presumably be derived from another common ancestor. Both of these 

 double lines, together with crinoids as a third line, would be en- 

 visaged as derived ultimately from a common dipleurula ancestry. 



