ASTACUS FLUVIATILIS. 287 



azygos nerve, which passes forward and upward and enters 

 the cerebral mass. I am inclined to think that this part of 

 the azygos nerve forms a portion of a fine plexus of nervous 

 filaments which pass from the cerebral ganglia backward to 

 the lining membrane of the carapace ; but the dissection of 

 these fine filaments, and the demonstration of their conti- 

 nuity, is a matter of no ordinary difficulty. 



The intestine is supplied by two nerves which arise from 

 the last abdominal ganglion, and unite into a single trunk, 

 from which small branches are given off backward, and two 

 principal ones forward, which supply the greater part of the 

 intestine. According to Brandt, the genitalia receive branches 

 of the fourth, fifth, and sixth thoracic ganglia. 



The only certainly known organs of sense in Astacus are 

 the eyes and the auditory organs. The eyes are seated at 

 the extremities of the ophthalmic peduncles, the integument 

 of the outer extremity of which becomes translucent over a 

 reniform space, and constitutes the corneal membrane. This 

 membrane is divided into a great number of minute quadri- 

 lateral facets, each of which corresponds with the base of a 

 crystalline cone. 1 



The upper face of the trihedral, proximal, and largest 

 joint of the antennule presents an oval space, covered by a 

 broad brush of complex hairs having their points all directed 

 inward. On cutting these hairs away close to their bases, 

 however, it is seen that they cover an aperture, wider above 

 than below, and about one-sixteenth of an inch long. The 

 hairs are attached to the outer lip of this aperture, and some 

 are directed so as to lie within the inner lip, but the majority 

 cover it. A good-sized bristle passes with great ease into 

 this aperture, and if the inner and outer walls of the basal 

 joint of the antennule be now removed, and the soft parts 

 carefully dissected away, the end of the bristle will be seen 

 to have'passed into a wide delicate sac about one-twelfth of 

 an inch long, which is attached by a narrower neck round 

 the aperture, the lips of which are continuous with its walls. 

 The sac is filled with minute sandy particles, suspended in a 

 mucous, dirty-looking fluid, and when emptied of these con- 

 tents a band, consisting of several lines of very fine hairs, 

 like those which guard the mouth of the sac, but more deli- 



* Mr. E. T. Newton's careful description of the eye of the Lobster in The 

 Quarterly Journal of Microscopical Science for 1875, to which I have referred 

 above, may be taken as a guide to the study of the minute structure of the eye 

 in the Crayfish. 



