2l6 GEOLOGICAL BIOLOGY. 



garded as of the nature of optic organs, but it is doubtful if 

 -any such organs were differentiated in the Cambrian type. 

 The nervous system in this type of animals at that time prob- 

 ably performed the function of co-ordination of organs. With 

 the differentiation of the alimentary canal there was probably 

 a specialization of cells for the particular function of these 

 several parts of the canal. The reproductive function had its 

 special organs differentiated, but they were as numerous as 

 the partitions of the body, and the elaboration of this system 

 in the Cambrian era had not proceeded far. 



The Annelids (which is in our classification a representa- 

 tive of the branch Vermes, but in Huxley's classification is 

 placed in a branch Annulosa, distinguished in some particu- 

 lars from the Arthropoda, but only as a sub-branch, the An- 

 arthropoda) are represented in the Cambrian. They are the 

 lowest or less differentiated type of the articulate mode of 

 body development. There is an elongation of the body, and 

 in the adult there is a definite division into segments (27) or 

 metamcrcs (somites repeated and arranged along a longitudinal 

 axis). A prominent distinction separating the Annelids from 

 the Articulata proper, as representatives of branch or class 

 groups, is the absence of jointed appendages articulated to 

 the somites in this division of Vermes, and Huxley recognized 

 this distinction in applying the name Anarthropoda (meaning 

 without joints) to the class, while the Crustacea, Insects, and 

 allied forms develop jointed and articulated appendages to the 

 somites. In this type of structure the differentiation of parts 

 in the first or radiate direction is completed in the strictly 

 bilateral symmetry. The function of motion has specialized 

 into definiteness of relation of the motions to the body a 

 longitudinal polarity. The direction from which supply of food 

 comes toward the body, or towards which the motor system 

 propels the body, is anterior (280); it is distinctly in front of 

 the mouth, while the other parts of the digestive system are 

 arranged definitely posterior (2 8&) to the mouth, along the longer 

 axis. The parts about the mouth are reduced to their small- 

 est number, and are determined definitely in relation to a 

 surface upon which progression takes place ; a ventral side (290) 

 and a dorsal side (29$) become thus distinguished. The 



