402 



CKUSTACEA. 



their investigations are of such great physiological importance*, thae 

 the following condensed account of their labours cannot be omitted in 

 this place. In Talitrus, every pair of ganglia consists of two separate 

 nuclei of nervous substance, united by a transverse band so disposed as 

 to bring them into communication with each other, while an anterior 

 and posterior nervous filament derived from each unites it with the 

 preceding and following ganglia of the same side of the body : even the 

 encephalic mass is composed of two lateral portions united by a cord 

 passing between them. All these pairs of ganglia (thirteen in number, 

 corresponding with the number of the segments of the body) are exact 

 counterparts of each other both in size and figure, so that none seems to 

 preponderate in energy over the rest ; but the anterior or encephalic 

 pair alone communicates with the eyes and antennae, the only organs of 

 the senses as yet discernible. 



(1026.) In Oniscus Asellus, a concentration of the elements com- 

 posing the nervous system above described is discernible ; and this is 

 found to be indicated by incipient approximation, which takes place 

 in two directions, one longitudinal, the other transverse. In the first 

 place, the entire number of pairs of ganglia is reduced to ten, three 

 pairs having become obliterated by coalescence ; and moreover, while 

 the central portions still consist of two lateral masses each, the first and 

 last pairs are united into single ganglia. As we rise to higher forms 

 the coalescence still proceeds; all the pairs of ganglia soon become 

 united in a transverse ^ 2Q5 



direction, and gradu- 

 ally the whole chain 

 becomes shorter by the 

 fusion of several pairs 

 into larger and more 

 powerful masses. 



(1027.) In the Crab 

 (which, from its ter- 

 restrial habits, holds 

 a position among the 

 Crustacea equivalent to 

 that which the Spiders 

 occupy among other Ar- 

 ticulata), this centrali- 

 zation is carried to the 

 utmost extent, and all 

 the abdominal and thoracic ganglia become agglomerated into one great 

 centre, from which nerves radiate to the parts of the mouth and in- 

 struments of locomotion (fig. 205). 



* " Itecherches Anatomiques sur le Systems Nerveux des Crustacea," Ann. des 

 Sci. Nat. t. xiv. 



Nervous system of the Crab : from the ventral aspect. 



