OLA.SSII'IOATIO:^f OF THE ANIMAL KTIfaDOM. 209 



gives rise to the alimentary canal, answers to the hypoblast, while 

 the rest corresponds with the epiblast. 



The large blastomeres which become enclosed between the 

 epiblast and hypoblast in the manner described, appear to serve 

 the purpose of a food-yelk ; and the space which they originally 

 occupied is eventually filled by a gelatinous connective tissue, which 

 possibly derives its origin from wandering cells of the epiblast. 



The Actinozoa and the Hydrozoa constitute the Coelenterata, 

 which are definitely characterized by the fact that, in all the higher 

 forms, the mesoblast is traversed by canals formed by diverticula of 

 the hypoblast, which permanently remain in continuity with the 

 alimentary cavity, and that, in the lower forms, the alimentary 

 cavity is prolonged into the coenosarc. They are usually said to 

 have a radiate symmetry ; but, even in the Actinics, there are traces 

 of bilaterality ; and in the Ctenophora the bilateral symmetry of 

 the adult is obvious. 



Parallel with these may be ranged an assemblage composed of 

 the Turbellaria, Eotifera, and Trematoda, the Nematoidea, Oli- 

 gochaeta, and Hirudinea, to which the name of ' Scolecimorpha' 

 may be applied. They are associated together by the closest 

 resemblances of structure, and present an even greater range 

 in grade of organization than the Coelenterata. The lower Hhab- 

 doccela come very close to the Infusoria (as close as the multicel- 

 lular to the unicellular Algse), and are but little superior to Hydra 

 in the degree of their organic differentiation, while in the land- 

 Planarise, the Trematoda, and the Nemertidse we have animals 

 which attain a considerable complexity and, in the case of many 

 Trematoda and of Lineus (Pilidium), undergo remarkable meta- 

 morphoses. Such forms as Dlnophilus appear to connect the 

 rhabdoccele Turbellaria with the Kotifera. The lower Nematoidea 

 are extremely simple, while the higher are considerably difi'eren- 

 tiated ; and, as Schneider has shown, they are connected with the 

 Turbellaria by such forms as Polygordius. The Oligochgeta and the 

 Hirudinea either belong to this division, or constitute a transitional 

 group between it and the Deuterostomata. In Lumhricus (and 

 apparently in Hirudo) there seems to be no doubt that the aperture 

 of invagination of the Gastrula becomes the mouth. According 

 to Kowalewsky, the mouth in Euaxes and Tubifex is of secondary 

 origin ; but its close resemblance to that of the earthworm and of the 

 leech embryos leads me to suspect that there must be some error 

 of interpretation here. On the other hand, it may be that these 



