436 ZOOLOGY sect. 



is not till we come to the Mesozoic Period that undoubted traces 

 of their existence are found. 



Affinities. — The presence of radial symmetry was formerly 

 regarded as involving a near relationship with the Ccelenterata, 

 which were grouped with the Echinodermata under the comprehen- 

 sive class-designation of Radiata (see section on the History of 

 Zoology). But, leaving out of account the presence of a bilateral 

 symmetry underlying and partly concealed by the radial, we are led 

 by a study of the anatomy of the various sj^stems of organs to the 

 conclusion that the Echinoderms are in no way closely or directly 

 related to the Ccelenterates. One very great and very important 

 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 — alimentary, vascular, and nervous — such as 

 occur in none of the Coelenterates, all of which exhibit ex- 

 treme 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 Echino- 

 derms 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 Echino- 

 derms 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 a more primitive bilateral arrangement of 

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



