30 ORGANIC DEPENDENCE AND DISEASE 



and clam. The rest are free (gastropods 39, pteropods 

 32 species). The sponges of the Cambrian are as yet in a 

 large measure undescribed but the material in the collec- 

 tions made by Dr. Walcott from the Burgess shale indicates 

 the great abundance of the silicious sponges, while they re- 

 tain a simplicity of form which is in contrast to the pro- 

 gressed species of the Devonian. 



With the foregoing we may contrast the great outstand- 

 ing army of independents — the Crustacea. Of the trilo- 

 bites there are 502 species and of the Eucrustacea, the prim- 

 itive shrimps, 89 species — together constituting one-half 

 the entire list of described species of the fauna. These 

 creatures were all elaborately innervated and highly loco- 

 motive throughout their entire life, and their anatomical 

 and functional structure was a very advanced attainment 

 in specialization. Such an enormous development of the 

 single type of structure represented by the trilobites, which 

 were here at the climax of their entire career on earth, gave 

 material and opportunity for different degrees of progress, 

 delay, decline and reversion, all of which are to be estimated 

 in the construction of a true classification of the great 

 group. No adequate conception of their specialization can 

 be obtained without a study of the restorations of their ven- 

 tral anatomy as shown by Neolenus, a late member of this 

 Cambrian or ''first fauna." This has been restored by 

 C. D. Walcott on the basis of specimens collected by him in 

 the Middle Cambrian of Burgess Pass, Alberta. The trilo- 

 bite has maintained throughout its individual (ontogenic) 

 and race (phylogenic) existence, complete freedom and full 

 locomotor efficiency. And if this is true of them it is a 

 fortiori true of the Eucrustacea^ of this fauna of which 



1 These Eucrnstaeea are creatures which to the casual observer show evident 

 relationship to the ' ' shrimps. " It is interesting to a paleontologist to observe 

 the unconscious solemnity with which biologists familiar alone with evident 

 structures in the vast group of living arthropods or jointed invertebrates, and 

 their classification, debate with themselves the position and affinities of these 



