PHYLOGENESIS IN CLASSIFICATION. 247 



entiated a permanent bilateral symmetry, which may be 

 considered as the final elaboration of the radial type of differ- 

 entiation which was dominant in the Coelenterata and Echino- 

 dermata. In those branches the antimeres are numerous, or 

 at least five, and the bilateral symmetry is only partially ex- 

 hibited, while in the Mollusca it is a dominant character in 

 the adults. In the Vermes and Arthropoda the bilaterality of 

 each somite is further differentiated in the longitudinal direc- 

 tion by the division into segments. Although the order of 

 rank exhibited by this mode of differentiation of parts would 

 lead us to look for Ccelentera with multiple radiate struc- 

 ture first, then the differentiation of the Medusa, then the 

 Echinodermata, with symmetrical radiation, then simple bi- 

 laterality of the Mollusca, to be followed by Vermes and 

 Arthropoda, we actually find that the Arthropoda are al- 

 ready abundant in the Cambrian, and in the Trilobites con- 

 stitute the dominant type of organism. 



Digestive System in the Mollusca As already suggested, in 

 the Mollusca the alimentary system is differentiated into a 

 digestive cavity. The products of digestion, finding their 

 way through the walls of the digestive cavity, are received 

 into a system of canals with a contractile reservoir, which is 

 the circulatory blood-system. In connection with the ali- 

 mentary system is also developed in the higher Mollusca, 

 and perhaps in all the classes, organs called nephridia, which 

 are apparently purificative in function, and are primitive 

 kidneys in differentiation. In all the Ccelomata gonads or 

 special reproductive organs are differentiated. 



Muscular, Nervous, and Motory Systems of Mollusca. Muscu- 

 lar tissue is distinctly differentiated, and also nervous tissue, 

 with the peculiar specialized functions of contractility and 

 sensibility. The nervous system consists of a gangliated ring 

 of nerve-fibres around the oesophagus, and in the higher 

 types of Mollusca special sense-organs are differentiated for 

 touch and sight. The motory system is developed in a char- 

 acteristic way in the several classes of Mollusca, in a foot 

 which is the most permanent and characteristic 'feature of the 

 branch. Perhaps the simplest way of expressing the rela- 

 tions of this foot to the structure of the organism and the 



