NERVOUS SYSTEM OP CRUSTACEA. 



383 



main 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 abdo- 

 minal region, though still distinct in the thorax. 

 , In following the ascending series of crustaceous animals, 

 we observe also an approximation of the remoter ganglia to- 

 wards those near the centre of the body: this tendency al- 

 ready shows itself in the shortening of the hinder part of 

 the nervous system of the Cymolhoa, as compared with the 

 Talitrus; and the concentration proceeds farther in other 

 tribes. In the Palemon, for example, most of the thoracic 

 ganglia, and in the Palinuriis (Fab.,) all of them, have co- 

 alesced into one large oval mass, perforated in the mid- 

 dle, and occupying the centre of the thorax; and, lastly, in 

 the Maia squinado, or Spider Crab (Fig. 441,"^) this mass 



• In this figure are seen the great thoracic g-anglion (n,) from which pro- 

 ceed the superior thoracic nerves (t,) those to the fore feet (f,) to the liiiulcr 



