CLASS OF CETJSTACEA. 435 



constitutes then a moveable apron placed on the lower surface 

 of the body between the limbs, 



564. The nervous system is composed of a double series 

 of ganglions, situated on the ventral aspect of the body near 

 the median line. Their number corresponds in general to 

 that of the distinct segments of which the body is composed, 

 and those of the first pair are always lodged in the head or in 

 front of the gullet, where they constitute a sort of brain 

 (Fig. 423, c) : but the arrangement of the ganglions of the 

 thorax and abdomen varies much; sometimes they are at 

 equal distances from each other, and form, with their cords of 

 communication, a chain extending from one extremity of the 

 body to the other ; sometimes they approach each other more 

 or less, and sometimes they are altogether reunited into 

 a mass situated towards the middle of the thorax (Fig. 



Fig. 424. Podopthalmus. 



423, t). It ought also to be observed, that the cen- 

 tralization of the nervous system becomes more and more 

 complete in proportion as the animal rises in the scale of 

 being or acquires a more elevated organization. Moreover, 

 all the Crustacea have very limited faculties, and none amongst 

 them present much interest in respect of their habits. The 

 eyes are formed pretty nearly as in insects. Sometimes they are 

 simple ; but generally they are compound or composite, and in 

 all the more perfect Crustacea they are carried on moveable pe- 

 duncles (Fig. 424), an arrangement not found in any of the 

 other divisions of this great class of articulated animals. 



In many Crustacea there exists also an organ of hearing, 

 situated at the base of the external antennae (Fig. 425), com- 

 posed of a small membrane resembling a membrana tym- 

 F F 2 



