— 10 — 



which runs parallel with the progressive consolidation and 

 unification of the segmented body as traced from the cœlen- 

 terate strobila through the cestode and the annelid to the 

 arthropods, and both body types appear finally to coalesce 

 in the vertebrates. 



The cœlenterates are solitary or colonial, and the colonial 

 type may, as in the siphonophores, become a more or less 

 unified entity. A similar dichotomous division of the body 

 structure, more or less marked, is observable in all the broader 

 animal groups indicated by increasing bodily complexity, 

 as well as in the plants. We cannot convincingly derive the 

 bodily structure of a sipunculid from that of a cestode, that 

 of an annelid from that of a nematode, that of an insect from 

 that of a sipunculid, or that of a brachiopod from that of 

 an annelid ; but if we can assume that fundamentally all 

 organisms are either simple or compound and that in each 

 group there is a strong tendency to segregation toward both 

 extremes the problem of tracing a logical phylogenetic develop- 

 mental line becomes much simplified. 



The third group of animals which can be assumed to have 

 arisen directly from the protozoans,, quite independently 

 of the sponges and cœlenterates, is characterized by the 

 possession of more or less perfected quadrilateral symmetry, 

 of a more or less developed centralized nervous system, and 

 of an excretory system ; there is no vascular system, no respi- 

 ratory system, no skeleton, and no true coelome, and hemo- 

 globin is never present in the tissues. This assemblage of 

 animal types, which includes the flat worms, the round worms, 

 the Acanthocephala, the rotifers, the Calyssozoa and the 

 polyzoans, is exceedingly heterogeneous on account of the 

 unorganized condition of the various organs and structures 

 of which the body in each of the component types is composed, 

 and hence of the lack of any definite correlation between them ; 

 but it should be borne in mind that the higher types, in which 

 the nervous system, the alimentary system, the nephridial 

 system, the generative system and the body cavity have been 

 organized and coordinated and reduced to a standard and 

 relatively inflexible type appear to be just as heterogeneous 

 when considered solely on the basis of the vascular, respiratory 

 and skeletal systems which first appear and gradually become 

 organized in them. 



