LESSON 86. J NERVOUS SYSTEM IN MAMMALIA AND MAN. 277 



tor in their function, with the exception of the sympathetic nerves 

 originating from the supra-oasophageal loop. In the Myriapods, 

 however, a large and well developed brain is found, to supply the in- 

 creased number of the organs of special sense, and to originate a well 

 formed sympathetic system. In Insects another and remarkable 

 change, of a strictly progressive character, awaits us. In their lar- 

 val, or infantile condition, their energies are in abeyance, and their 

 organs of special sense are few, compared to their condition in the 

 perfect insect ; the superior portion of the brain, therefore, is much 

 smaller than it is hereafter destined to become. The energy, activi- 

 ty, and great irritability of the perfect Insect, contrasts remarkably 

 with the dull, sluggish, almost impassible life of the same animal in 

 its young condition. In a caterpillar, any one segment or ring of the 

 body (except the head) is like all the rest, and has the same func- 

 tions, even of locomotion, to perform hence the necessity of a gan- 

 glionic enlargement, or little brain in each segment, to keep up, or 

 maintain, the nervous force necessary for the due performance of 

 their functions respectively. The brain is much larger than their 

 inferior portions in the perfect Cockroach (Fig. 417), and Mantis 

 religiosa (Fig. 419). 



1246. In the bivalve-mollusca, we find a creature in which the 

 nutritive system appears to be developed at the expense of all the 

 other organs ; consequently there is little special sense, and no need 

 of nerves to supply organs that do not exist. 



1247. Destitute of vision, optic nerves are unnecessary; devoid of 

 smell and taste, there are neither gustatory nor olfactory nerves, and 

 the brain (or that which is analogous to it, as some authors suppose) is 

 reduced to the two cephalic ganglions, which are placed on either side 

 of the oesophagus, and much smaller in size than the branchial gan- 

 glia. From all analogy of the distribution of nerves from special 

 centres, and the organs they supply, the cephalic ganglia of the bi- 

 valve mollusca appear to constitute, not the superior, but only the 

 inferior surface of the brain. Headless, and entirely destitute of 

 organs of special sense, there is no need of a supra-cesophageal gan- 

 glion, and none such is developed; the branchial ganglions, with 

 their mixed function, appear to be the most important. The same re- 

 marks apply to the still lower Tunicata and Terebratulce. 



1248. But in the Slugs and Snails, whether naked or testaceous 

 (testa, a shell), we find animals endowed with that superior organiza- 

 tion that renders a higher development, no less than a local position 

 and concentration of the brain, imperatively necessary. With a 



