xii.] THE FRESH-WATER CRAYFISH. 137 



sural cords, which pass on each side of the gullet, with 

 a larger ganglionic mass, which is called the subcesophageal 

 ganglion. This occupies the region of the hinder part of 

 the cephalon and the anterior part of the thorax, and gives 

 off nerves to the maxillae and the three pair of maxillipeds. 

 Five other ganglia lie in the five somites which bear the 

 chelae and the ambulatory limbs, and there is one for each 

 abdominal somite, the last of these being the largest of the 

 six. 



The longitudinal commissures between the abdominal 

 ganglia are single ; but, in the thorax, the commissures are 

 double, and the ganglia themselves shew more or less evi- 

 dent indications of being double. And there is reason to 

 believe that these thirteen apparent ganglia really represent 

 twenty pairs of primitive ganglia, one pair for each somite ; 

 the three pairs of praeoral ganglia having coalesced into the 

 brain ; and the five which follow the mouth having united 

 into the subcesophageal mass. 



The only organs of special sense which are recognizable 

 in the Lobster and Crayfish are eyes and auditory organs. 



The eyes are situated at the extremities of the eyestalks, 

 or ophthalmites, which represent the first pair of appendages 

 of the head. The rounded end of the eyestalk presents a 

 clear, smooth area of somewhat crescentic form, divided into 

 a great number of small four-sided facets. This area cor- 

 responds with the cornea, which is simply the ordinary 

 chitinous layer of the integument become transparent. 

 The inner face of each facet of the cornea corresponds with 

 the outer end of an elongated transparent slightly conical 

 body the crystalline cone the inner end of which passes into 

 a relatively long and slender connective rod, by which it is 

 united with a spindle-shaped transversely striated body 



