PRIMARY DIVISIONS OP THE ANIMAL KINGDOM. o 



French naturalist that has been dismembered into distinct groups 

 in consequence of our rapidly increasing knowledge of the 

 diverse structure of these simpler forms of life. A comparison 

 of the views of the most recent authorities, however, shows 

 considerable divergence in their estimates of the systematic 

 importance of structural peculiarities. Ray Lankester, for 

 instance, admits no less than ten primary groups or subking- 

 doms below the mollusca. It matters little to the conchological 

 student, however, whether some of the vermes be considered 

 independent subkingdoms or not. Classification is necessarily 

 based upon half-truths, because the values of structural differ- 

 ences are not demonstrably equivalent throughout, and because 

 lines of affiliation co-exist with those of difi'erentiation in most 

 cases. Nature can only be artificially forced into the systematic 

 relations of class, order, family, genus, etc. Still, these divisions 

 have a conventional value, constantly varying according to the 

 mental standpoint from which they are studied. 



The Tunicata have been thought to form a connective link 

 with the vertebrates through their most degraded member, 

 Amphioxus, and Gegenbauer * makes them an independent 

 " phylum " between Mollusca and Vertebrata ; whilst E. Ray 

 Lankester actually unites them with the latter group. The 

 want of co-ordination of structural peculiarities in the so-called 

 types of animals is, of course, considered important evidence 

 for the development or derivative theorj", and it has become 

 fashionable to construct ancestral trees, bearing branches and 

 leaves representative of the several animal types and their 

 derivative relationships. As an example, Gegenbauer places 

 Protozoa at the root of his tree, whence ascends a lateral, 

 limbless trunk, terminating in Coelenterata, whilst the main 

 trunk rises vertically to Vermes, where it divides into branches 

 bearing consecutively (from left to right), -Mollusca, Vertebrata, 

 Arthropoda, Brachiopoda, Echinodermata. Tunicata is a 

 branchlet, proceeding from Vertebrata, and leaning towards 

 Mollusca. The author of this structure speaks thus of his 

 work : " These divisions represent in a general way separate 

 branches of the pedigree of animals, and each of them contains 

 higher and lower forms in various proportion. But the degree 

 and extent to which their organization is developed is different 

 in each of them. The divergence of organization expressed in 

 each division is indicated by their relations to one another, and 

 it shows us how the lower forms of the higher phyla may have 

 started from the lower phyla." f It is sufficient, perhaps, to 

 say here that the consanguinity indicated by the common 

 characters of existing primary types, has not been in any 



* Elements of Comparative Anatomy. f Gegenbauer, Anat., 70. 



