NERVOUS SYSTEM OF CRUSTACEA. 480 



united into single masses, while those in the thoracic 

 region are still double. In the Cymolhoa (Fab.), 

 which belongs to the family of Onisciis, there is the 

 appearance of a single chain of ganglia, those on 

 the one side having coalesced with those on the 

 other; each pair composing a single ganglion, 

 situated in the middle line ; while the longitudinal 

 cords which connect them still remain double, as is 

 shown in Fig. 440, which represents the interior of 

 this crustaceous animal, nearly of the natural size. 

 But in the higher orders of Crustacea, as in the 

 Lobster, these longitudinal cords are themselves 

 united in the abdominal region, though still distinct 

 in the thorax. 



In following the ascending series of crustaceous 

 animals, we observe also an approximation of the 

 remoter ganglia towards those near the centre of 

 the body : this tendency already shows itself in 

 the shortening of the hinder part of the nervous 

 system of the Cymothoa, as compared with the 

 Talitrus; and the concentration proceeds farther 

 in other tribes. In the Palemou, for example, most 

 of the thoracic ganglia, and in the PaUmirus (Fab.), 

 all of them, have coalesced into one large oval mass, 

 perforated in the middle, and occupying the centre 

 of the thorax ; and lastly, in the 3Iaia sqainado, or 

 Spider Crab (Fig. 441),* this mass acquires still 



* In this figure are seen the great thoracic ganglion (b), from 

 which proceed the superior thoracic nerves (r), those to tlie fore 

 feet (f), to the hinder feet(f), and the abdominal nervous trunk (n) ; 

 the cephalic ganghon (c), communicating by means of two nervous 

 cords (o), which surround the oesophagus and entrance into the 

 stomach (s), with the thoracic ganglion (b) ; and sending off the 

 optic nerve (e) to the eyes (e), and the motor nerves (m), to the 

 muscles of those organs; and also the nerves (a) to the internal 

 antennae, and the nerves (x) to the external antennae (a). 



