THE GENERAL ANIMAL FUNCTIONS. 



73 



in a ring about the mouth, or gullet, with or without addi- 

 tional bands of nervous tissue containing cells passing radially 

 from it (as in echinoderms and some ccelenterates ; see Fig. 

 35). In the higher invertebrates this process of concentration 

 continues and the ganglionic cells are collected into two or 

 more ganglia connected by nerve fibres (commissures). Usu- 



FIG. 37- 



FIG. 37. Arrangement of the nervous material in the anterior end of an Oligochetc 

 Worm, seen in profile. That part of the body wall nearest the observer is supposed to 

 be removed, a, anterior; b.w., body wall; g, dorsal ganglia (brain); g', ventral chain 

 of ganglia; n, nerve ring around the pharynx; o, mouth; p, pharynx. 



FIG. 38. The central nervous system in a Leech. Lettering as in Fig. 37. 



Questions on figures 36, 37, and 38. How is the nervous matter re- 

 lated to the digestive tract and to the animal as a whole in all of these 

 figures? Compare with figures in other texts. Is there any apparent 

 correlation between the form, symmetry or segmentation of the animal 

 and the arrangement of the nervous material? Can you state your con- 

 clusion as a law? 



ally a pair of ganglia occurs in the region of the mouth, and 

 dorsal thereto (e. g., clam, Fig. 36). In segmented forms, 

 as the earthworm and crayfish, there is also a series of gang- 

 lia connected by fibres, ventral to the digestive tract. This 

 chain is in turn connected with the dorsal ganglia by a loop 

 of fibres passing round the oesophagus (Figs. 37 and 38). 



