NO. 3 



INSECT HEAD SNOUGRASS 



17 



masses, it will be necessary to understand next the essential features 

 in the evolution of the central nervous system in segmented animals. 



The annelids, as already noted, have a ganglionic nerve mass lying 

 in the anterior part of the body, before or above the stomodeum, 

 which takes its origin from the ectodermal apical plate of the pro- 

 stomium (fig. 7 F, Ap) . This, the most primitive brain of the annelid- 



Arc 



a,.NC 



Com 



Fig. 12. — Diagrams suggesting the evolution of a central nervous S3'stem of 

 annelid-arthropod type of structure. 



A, theoretical structure of nervous system in an unsegmented pre-annelid 

 form, consisting of a prostomial archicerebrum (Arc), and of two ventro- 

 lateral nerve cords (NC), connected medially by transverse nerves, and giving 

 off nerves laterally to body wall and internal organs. Nerve cells dififused along 

 the cords. 



B, simple nervous system of the ladder type in a segmented animal. The 

 nerve cells aggregated into segmental groups, or ganglia (Gng), along the cords ; 

 the intervening parts of cords converted into connectives (Con), and the 

 transverse ventral nerves forming commissures (Com) between the ganglia. 



C, the segmental pairs of ganglia united into compound ganglia of a median 

 ventral nerve cord (J'NC), in which the first, or suboesophageal, ganglion 

 (SocGng) is postoral, and connected with archicerebrum of prostomium (Arc) 

 by connectives (CocCon^ encircling the mouth (Mth). 



arthropod series (fig. 12 A, Arc), has been named by Lankester 

 (1881) the archicerebrum (a happy, though mismated union of lin- 

 guistic elements). In the trochophore, a pair of dorsal and a pair of 

 ventral nerves (fig. 7 F, DNv, VNv) extend backward from the 

 apical plate, but in the adult worm and in arthropods only the nerves 

 of the ventral pair are retained. In the more primitive condition, the 

 two ventral nerve strands have a latero-ventral position (fig. 12 A, 



