ix PHYLUM ECHINODERMATA 427 



difference between the two phyla consists in the presence in the 

 Echinodermata of an extensive ccelome or body-cavity lined by 

 mesodermal epithelium between the alimentary canal and the 

 body-wall. In addition to this the Echinoderms are characterised 

 by the possession of highly elaborated systems of organs alimen- 

 tary, vascular, and nervous such as occur in none of the Ccelen- 

 terates, all of which exhibit extreme simplicity in their internal 

 structure. A further point of difference, not perhaps of so much 

 importance, is the absence in the Echinoderms of any tendency to 

 form colonies of zooids by asexual multiplication by means of buds : 

 all Echinoderms are simple, i.e. non-colonial, animals, and each of 

 them is developed, save in certain very exceptional cases, as a 

 result of a sexual process from an impregnated ovum. In spite, 

 then, of the radial symmetry, we are forced to the conclusion that 

 the Echinodermata are not more nearly related to the Ccelenterata 

 than to some of the groups of Worms. They are, in fact, a singularly 

 isolated group, and we look in vain among the known members, 

 living and fossil, of other phyla for any really close allies. The 

 intermediate forms whatever they may have been like between 

 the Echinoderms and other groups have become extinct, and have 

 left no remains in the form of fossils, or such remains have not yet 

 been discovered. So difficult has it been found to connect the 

 Echinoderms with other animal types that it has even been proposed 

 to regard an Echinoderm as a radially arranged colony of zooids 

 connected together centrally, each ray being a zooid equivalent 

 to an entire simple worm-like animal. But the history of the 

 development is entirely at variance with such a view. 



Whatever may have been the group of animals from which the 

 Echinodermata were developed, there is every probability that it 

 was a group with bilateral and not radial symmetry. The radial 

 symmetry is evidently, as has already been pointed out, of a 

 secondary character ; it is only assumed at a comparatively late 

 period of development, and even in the adult condition it does 

 not completely disguise an underlying bilateral arrangement of 

 the parts. Accordingly, within the phylum itself, it is reasonable 

 to regard those classes as the more ancient which have the radial 

 symmetry less completely developed. Again, the free condition 

 which characterises all existing Echinoderms, with the exception 

 of a few Crinoids, is probably less primitive than the attached, 

 since in other phyla the radial symmetry is co-ordinated with, 

 and seems to be developed on account of, a fixed, usually stalked 

 condition. Probably, then, stalked Echinoderms were the pro- 

 genitors of the existing free forms, and these were preceded by 

 primitive free forms with pronounced bilateral symmetry. It 

 appears to be most probable that this ancestral form possessed the 

 most essential features of the diplewrula larva (p. 422) ; i.e., that 

 it was a bilaterally symmetrical form with a pre-oral lobe, simple 



