258 INVERTEBRATE ZOOLOGY 



nings of metamerism are readily observable in the flatworms. 

 Regular arrangement of the lateral diverticula from the digestive 

 tract and orderly disposition of the gonads between these divertic- 

 ula indicate preparations for a segmentation of the body before 

 the septa make their appearance as in the annelids. 



Echinoderma. — The echinoderms offer but few clews to an 

 explanation of their origin. Their radial symmetry is not 

 primitive, and, since the larvae are bilaterally symmetrical 

 throughout, it seems probable that the members of this group 

 have evolved from a bilaterally symmetrical coelomate ancestor. 

 MacBride has emphasized the fact that echinoderm larvae 

 represent a primitive type not directly related to the trochophore. 

 Echinoderm larvae show more resemblance to the tornaria larva 

 of Balanoglossus than to any of the lower Metazoa. In some 

 instances this fact has been utilized in tracing a possibile ancestry 

 of the vertebrates through a line which passes back through the 

 Enteropneusta (Balanoglossus) to a primitive type from which 

 the echinoderms and the Enteropneusta have arisen. 



Arthropoda. — The arthropods have many points in their 

 morphology which are likewise shared by the annelids. Even 

 the parapodia of the annelids are not so markedly different from 

 the foliaceous appendages of the lower Crustacea. The Onyco- 

 phora (Peripatus) were for a long time considered as worms, but 

 closer investigation revealed characters which have been looked 

 upon as diagnostic of the Arthropoda. Standing as they do, mid- 

 way between the arthropods and the annelids, the Onycophora 

 show a transition from one group to the other which may well be 

 taken as indicative of a wormlike ancestry for the arthropods. 



In this phylum there arises an interesting conflict between the 

 evidences presented by embryology and morphology. On the 

 basis of morphological study the arthropods seem to have their 

 closest affinities with the elongate annelidlike forms, but in 

 development all of the lower Crustacea pass through a character- 

 istic larval form known as the nauplius. This larval form is 

 very short and bears but three pairs of appendages. There is no 

 conclusive method of weighing the relative merits of the two 

 lines of evidence. It seems, however, that the nauplius might 

 be an adaptation to a free-swimming existence and thus may 

 have undergone changes a full record of which has not been 

 retained in the much shortened history of the race which is pre- 

 sented by ontogeny. Some who see in the nauplius a significant 



