yo Descriptive Zoology. 



wall. This leads into a thin sac, which serves as a bladder, 

 and it in turn opens by a duct to the exterior, through the 

 apex of a hard, white, conical papilla, on the ventral surface 

 of the base of each antenna. 



It is worthy of notice that the current of water coming 

 from the gills passes directly by this other exit of waste, 

 so that one stream carries away all the waste products, 

 avoiding duplication of machinery. 



The Nervous System of the Crayfish. The nervous sys- 

 tem consists of a nerve cord which lies on the floor of the 

 body cavity, extending the whole length of the body in the 

 middle line. It is a white cord, composed of a ganglion 

 for each segment, connected in line by nerve fibers. The 

 cord is really double, but the two rows of ganglions have 

 consolidated, so that there appears to be but one row of 

 ganglions. The cord itself, between the successive gan- 

 glions, while apparently single, is actually double. And in 

 two places the double nature of the cord is manifest. The 

 two parts of the cord pass on the right and left of the 

 gullet, forming what is called the esophageal ring, or 

 esophageal collar ; again, the sternal artery passes between 

 the two halves in the interval between two of the gan- 

 glions. Though there is a ganglion for each segment, 

 there is a consolidation, so that there are but thirteen dis- 

 tinct ganglions for the twenty (or twenty-one) segments. 

 The first ganglion, called the brain or the cerebral ganglion, 

 is the result of the fusion of three pairs of ganglions. Back 

 of the gullet, and connected to the brain by the two com- 

 missures passing on either side of the gullet, is a large 

 ganglion, which is evidently composed of the ganglions of 

 the last three cephalic segments united with the ganglions 

 of the first three segments of the thorax. Following this 

 are five more distinct ganglions in the thorax and six in the 



