242 ANIMAL PHYSIOLOGY. [LfcSSON 73. 



centre whence voluntary impulse may be directed along the non- 

 ganglionic tracts of the nervous axis, and to which ordinary sensa- 

 tion may be transferred by similarly uninterrupted nervous filaments. 

 So, too, we find the function of the cerebellum simulated by the large 

 sub-cesophageal mass, which originating the nerves analogous to the 

 fifth pair in man, and the higher mammalia, for the masticating 

 organs and other parts of the head, may be regarded as analogous to 

 the base of the brain. 



1100. The stomato-gastrie (sympathetic) nerves, complete this 

 complicated system. 



LESSON LXXIII. 



NERVOUS SYSTEM IN CEUSTACEA. 



1101. In the numerous and diversified class of Crustacea, every 

 condition of the nervous system is met with, from that of the lowest 

 Annellide, or the earliest larval state, where scarcely a filament is yet 

 perceptible in the place of the nervous columns, to that concentration 

 of the nervous ganglia around the oesophagus, which connects the 

 highest Articulata with the Mollusca. 



1102. The supra-oesophageal ganglia (brain) are generally larger 

 than those of Arachnida, and smaller than those of Insects ; they are, 

 for the most part, united into a single cerebral ganglion, devoted 

 chiefly to the large organs of the senses, and their nerves unite with 

 the sympathetic, as in Insects. 



1103. The ganglia of the cephalo-thorax (head-chest, which in 

 these animals is soldered into one piece) vary much in their number, 

 magnitude, and degree of approximation, according to the form of 

 that part of the trunk, and the size of the several pairs of legs. 



1104. Many of the lower Crustacea have the segments nearly 

 equally developed from the anterior to the posterior extremity of the 

 trunk, and this equal development is seen also in the nervous co- 

 lumns and ganglia of these segments, as shown in the subjoined figure 

 of the nervous system of Talitrus locusta, or Sand-hopper, so com- 

 mon on the sea-shore (Fig. 366). 



1105. The slender longitudinal columns, and the minute ganglia 

 along their course, here remain distinctly separated from each other 

 by a small space on the median plain ; the ganglia are nearly of the 

 same size from the first pair (a), above the oesophagus (&), to the 

 caudal pair (c), and the pairs are almost equidistant along the whole 



