26 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 97 



such sensory organs in the anneUds, the so-called "ventral organs" of 

 the Onychophora, from which the nerve cords are differentiated, sug- 

 gest that the latter took their origin from ectodermal structures of 

 some kind. 



The final connection between the brain and the ventral nerve cords, 

 according to Kleinenberg and Meyer, is established by fibers that grow 

 forward from the first ventral ganglia (fig. 9 B) and unite with the 

 lateral nerve trunks (fig. 8 B, Vdn) extending posteriorly from the 

 arms («") of the cerebral commissure. The union thus formed pro- 

 duces the stomodaeal (circumoesophageal) connectives, through which 

 the prostomial and somatic nerve centers are unified in the definitive 

 nervous system. 



The peripheral subcutaneous nervous system of the adult worm is 

 developed directly from scattered neurocytes of the ectoderm. To 

 this system Kleinenberg ascribes the parapodial ganglia (fig. 9 D, 

 PdGng), which, he says, are formed quite independently of the central 

 system by groups of ectodermal neurocytes situated mesad of the 

 parapodial bases. Secondarily, the parapodial ganglia send connecting 

 nerves to the ventral nerve cords. 



IV. THE ADULT ANNELID 



The final development of the adult polychaete annelid from the 

 larva depends upon the histogenic activity in the zone of undiffer- 

 entiated cells situated between the last larval somite and the pygidium 

 (fig. II B, ZG). Within this zone of growth is generated anteriorly 

 a series of secondary postlarval somites (C, D), which does not repre- 

 sent an extension of the body, but an expansion of a small part of it, 

 since the new somites are interpolated between the primarily seg- 

 mented larval body and the pygidium. The more anterior somites of 

 the new series, being those first formed, are the first to acquire the 

 mature structure. The teloblastic growth-process is the same whether 

 the larva is a typical trochophore (fig. 12 B, D), or one more nearly 

 resembling the adult worm (E, F, G), but in the first case a greater 

 degree of metamorphosis accompanies the formation of the new 

 somites. Hence, though we may eliminate the trochophore from our 

 concept of the primitive annelid, we cannot dismiss the secondary 

 formation of the teloblastic somites as a purely ontogenetic process — 

 it must be explained in terms of phylogeny. 



THE TELOBLASTIC, OR POSTLARVAL, SOMITES 



The zone of growth, as described by Lillie (1906) in Arenicola 

 cristata, is a mass of large clear mesodermal and ectodermal cells, 



