THE EVOLUTION OF INVERTEBRATES 299 



The phylum Arthropoda is pretty clearly distinct from 

 other phyla. Its advance over the Annulata consists in the 

 tendency of the somites to be fewer and more definite in 

 number. The somites are grouped in two or three regions; 

 in certain regions the somites are fused, an indication of still 

 greater differentiation. All the appendages are segmented. 

 The dorsal blood-vessel is more differentiated than other 

 parts of the circulatory system, and in some classes it is a 

 clearly defined one-chambered heart. Sense-organs, especially 

 eyes, reach a condition of great complexity in comparison 

 with what is found in lower phyla. 



If division into somites is an indication of advance, the 

 loss of it would seem to be an indication of degeneration, but 

 we have no proof that the ancestors of the Mollusca had 

 bodies which showed metamerism. Their development was 

 evidently in a different direction from that of the Annulata 

 and the Arthropoda. The factors which seem to place Mol- 

 lusca higher in the system are greater centralization of the 

 nervous system in a miniature "brain," and the possession 

 of a three-chambered heart, two auricles and one ventricle. 



INVERTEBRATE PHYLA AND CLASSES 



The subjoined list of the classes discussed in the first 

 twenty-one chapters will be of service in recalling the rela- 

 tive position of the phyla in the system of classification. 



I. PROTOZOA 



Class 1. Sarcodina; example, Amoeba protens. 

 Class 2. Mastigophora ; example, Euglena viridis. 

 Class 3. Sporozoa; example, Plasmodium malariae. 

 Class 4. Infusoria ; example, Paramoecium caudatum. 



II. PORIFERA 



Class 1. Porifera; example, Heteromeyenia ryderi. 



