NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS 21 



and as genital outlets. Finally, in the Onychophora and the Arthrop- 

 oda, the coelomic sacs have been divided into gonadial compartments 

 and nephridial compartments, which have become reduced in size and 

 limited to restricted parts of the body, with the result that the 

 haemocoele is restored as the functional body cavity. 



III. DEVELOPMENT OF THE ANNELID NERVOUS SYSTEM 



The annelids and the arthropods undoubtedly have a closer bond 

 of union in the structure of the nervous system than in any other 

 feature of their organization, except metamerism itself. The definitive 

 central nervous system of the polychaete annelids is developed from 

 two distinct sources, one located in the prostomium, or episphere of 

 the trochophoral larva, the other in the somatic region, or hyposphere 

 of the larva. From the first is produced the brain ; from the second, 

 the ventral nerve cords. The nervous system of the trochophore con- 

 sists of ganglionic centers in the prostomium connected by circular 

 and radial nerve tracts, from which trunks proceed into the hypo- 

 sphere (fig. 4 A). This primary system centering in the prostomium 

 must represent the primitive neural system of the unsegmented an- 

 cestors of the annelids, adapted to the structure of the trochophoral 

 larva, and is probably congenetic in its origin with the nervous system 

 of the Platyhelminthes. The segmentally ganglionated ventral nerve 

 cords of the postoral region of the trunk are correlated in their 

 development with the development of body metamerism ; they pertain, 

 therefore, to a later stage of evolution, and have no homologues in 

 the unsegmented worms. The definitive connection between the pro- 

 stomial and somatic parts of the nervous system is established secon- 

 darily in the ontogeny of the polychaetes, but in the oligochaetes the 

 two parts are said to be continuous from their inception. The funda- 

 mental structure of the somatic nervous system of the articulate 

 animals is an adaptation to the function of regulating the muscular 

 mechanism of metameric body movement; the prostomial system is 

 primarily sensory in function, except insofar as it controls the move- 

 ments of prostomial appendages. 



The phylogenetic origin of the articulate nervous system can prob- 

 ably be interpreted very closely from the development of the neural 

 elements in the trochophore larva of the polychaete annelids, and must 

 have been about as follows : The primary neurocytes were probably 

 sensory cells of the ectoderm closely associated with the primary myo- 

 cytes, and were thus, at first, both receptive and motor in function. 

 As the muscular system became elaborated, however, the primary 

 neurocytes were withdrawn to the inner surface of the ectoderm. 



