3G COMPARATIVE ANATOMY. 



of various sizes, united by nerve-cords, termed ' commissural 

 cords,' and occupies the ventral surface of the body. 



The total number of ganglia is thirteen. Of these, the first 

 is the pra3-oral or cephalic ganglion, from which are derived the 

 nerves supplied to the eyes, antennae, and antennules. Black 

 bristles have been placed under these nerves. The remaining 

 and post-oral twelve are equally divided between the thoracic 

 and abdominal segments. The first post-oral ganglion (formed 

 by the fusion of six embryonic ganglia) is of large size, and 

 supplies six pairs of appendages, viz. the mandibles, two pairs 

 of maxillaB, and three pairs of ' maxillipeds.' The five succeed- 

 ing ganglia, smaller in size, and comparatively close together, 

 supply the five pairs of ambulatory legs. The abdominal 

 ganglia, still smaller in size and separated by greater inter- 

 vals, supply the powerful abdominal muscles, and the segmental 

 appendages. The last of these ganglia is of larger size than the 

 rest, and gives off a leash of nerves to supply not only its own 

 segment and appendages, but the 'telson.' The longitudinal 

 commissural cord is double in the segment anterior to the abdo- 

 men, but in the latter region it is reduced by fusion to a single 

 thread. Between some of the abdominal ganglia may be seen 

 the 'nervi transversi.' These are small nerves given off from 

 the longitudinal cord, at a point about midway between the 

 ganglia. 



52. A Cray- Fish (Astacus fluviatilis), dissected to shew 

 the male generative system. 



The whole of the heart and the greater part of the liver 

 have been removed. The testes are situated immediately 

 behind the posterior end of the stomach, and consist of two 

 lateral halves, which posteriorly fuse into a long thin mass, 

 situated in the median line. On either side the 'vasa defe- 

 rentia' quit the gland at the junction of its three apparent 

 lobes, and after describing a mass of convolutions on either side 

 of the hinder part of the thoracic cavity, terminate by a small 

 prominent orifice on the basal joint of the last pair of thoracic 

 legs. A slip of blue paper has been placed under the terminal 

 segment of each ' vas defercns.' 



Rolleston, p. 106, sq. 



