THE CRAYFISH. 41 



PLATE X. 



THE CRAYFISH. NERVOUS SYSTEM AND SENSE ORGANS. THE HISTOLOGY OF THE 



EXOSKELETON. THE KENAL AND REPRODUCTIVE ORGANS, AND DEVELOPMENT. 



FIG. 1. The nervous system, dissected from above. 



The dorso-lateral skeleton, together with the large muscles and the viscera exclusive of 

 the alimentary canal were removed ; the intestine was severed in order that the visceral 

 nerves might be seen. 



On the right side the endophragmal system was left untouched, while on the left it and 

 all else were removed, to the level of the articulations of the appendages ; these are indicated 

 by a shade. The left half of the rostrum was dissected away, and the eye-stalk opened 

 up. <?. x 2. 



This figure illustrates the relations borne by the nervous system to the body seginents ; 

 having made that out, the student is recommended to remove the whole nerve-chain and 

 examine it under water on a black surface. 



FIG. II. Median longitudinal section, through the eye stalk. Chromic acid pre- 

 paration. A. 2. 



The zones of the rod and spindle layer shaded over, are those at which pigment is most 

 fully developed. 



FIG. III. A portion of the cornea, stripped off and examined under water. D. 3. 

 This figure is an accurate drawing of a portion in which the ordinary typical square 

 facets graduate into hexagonal ones, both types being present in Astacus. 



FIG. IV. Three of the crystalline bodies from a fresh eye, teased up iu water. 

 The middle one is drawn en face, the right hand one from the side ; the pigment is 

 indicated in connection with the left hand one alone. D. 3. 



FIG. V. The left antennule, dissected in situ from within. 



The inner wall was removed to the level of the auditory sac, which was in part opened 

 up. x 6. 



FIG. VI. Two of the exopoditic sensory hairs of the above, drawn in the natural 

 position, the exopodite being in life directed upwards as in Fig. V. (Compare Fig. I., 

 Plate VIII.) D. 3. 



FIG. VII. A small portion of the auditory sac, viewed from within, after removal from 

 the body. 



The sandy particles functional as otoliths, are indicated in the upper portion of the 

 figure alone. The line of attachment of the auditory hairs coincides with that described by 

 the nerve, n.au., of Fig. V. D. 3. 







