478 ZOOLOGY. 



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. 509, 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 (Pig. 509, t). 

 It oiio-nt also to be observed, that the centralization of the 



1 ) 



Fig. 510. Podophthalmus ; Stalk-eyed Crab. 



nervous system becomes more and more complete in propor- 

 tion as the animal rises in the scale of being or acquires a 

 more elevated organization. Moreover, ail 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 peduncles 

 (Fig. 510), 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. 511), com- 

 posed of a small membrane resembling a membrana tym- 

 pani, above which is found a kind of vestibule filled with 

 liquid, and enclosing the termination of a special nerve. 

 Nothing is for certain known respecting the faculties of smell 

 and taste in these animals. 



