NERVOUS SYSTEM OF CRUSTACEA. 



545 



f 



and lastly, in the Maia sqninado, or Spider 

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

 compactness, assumes a more globular form, and 

 has no central perforation. 



These different forms of structure are also 

 exemplified in the progress of the developement 



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

 which proceed the superior thoracic nerves (t), those to the 

 fore feet (f), to the hinder feet (f), and the abdominal nervous 

 trunk (n) ; the cephalic ganglion (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 

 antennse (a). 



VOL. II. N N 



U, 



