8o ZOOLOGY 



107. Arrangement of the Central Nervous System. — The 

 ganglion cells composing the nervous system may be so scat- 

 tered through the superficial layers as scarcely to deserve the 

 name "central" (Hydra). The nerve cells may be arranged 

 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. 

 37). 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). Usually 

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

 to it (e.g., clam, Fig. 38). In segmented forms, as the earth- 

 worm and crayiish, there is also a series of ganglia 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 esophagus (Figs. 39 and 40). In vertebrates the cen- 

 tral nervous system consists primarily of a tube with thick 

 nervous walls — the spinal cord — which may be specially en- 

 larged and thickened at the anterior end to produce the brain 

 (Fig. 172). From the various parts of this cord the nerves take 

 their origin, and run to all parts of the body. 



108. The Peripheral Nervous System: Sense Organs. — 

 We know by experimentation that in the lowest animals even, 

 or for that matter, in protoplasm, certain external conditions 

 produce definite responses or changes. We also know that 

 these external happenings and their responses, in our own 

 case, are accompanied by certain sensations, as touch or taste. 

 By inference, both from the nature of the response and from 

 the structure of organs, we reach the conclusion that the lower 

 vertebrates and higher invertebrates experience sensations in 

 some degree similar to our own. The classes of possible stimuli 

 have already been mentioned (§21). Those producing in us 

 definite sensations are: simple contact stimuli, producing the 

 sensation of touch and pressure; vibratory contacts, giving rise 

 to hearing and temperature sensations; gravity, giving us our 

 sense of position in respect to th^ pull of gravity; chemical ac- 

 tions, making possible sensations of taste and smell; ethereal 

 vibrations, producing the sensation of light. In the lowest 



