394 THE CRAYFISH CHAP. 



antennule under the low power, and note the tufts of 

 spatula-like " olfactory " setce on the ventral surface. Sketch. 



3. " Auditory " organ (statocyst). Carefully cut away the 

 convex ventral side of the basal segment of the antennule 

 with scissors, so as to expose the statocyst. Cut this out 

 and place it on a slide, carefully removing the muscles 

 surrounding it, as well as the setae around its aperture. 

 Note the contained grains of sand, and then wash them 

 away. Stain with magenta and mount in glycerine, 

 flattening the sac out with a cover-glass. Note that the 

 sac is an involution of the integument lined by cuticle, and 

 that it contains simple jointed sensory setcs of various sizes, 

 arranged in rows, and that branches of the antennulary 

 nerve run up the stem of each seta. Sketch. 



4. The eyes. Remove one of the eye-stalks, and note 

 the apparently black, uncalcified, oval portion of the 

 cuticle (cornea) at its distal end. Strip this off, and note 

 that it is transparent. Then wash off any pigment which 

 may have come away with it and mount in water. Observe 

 the corneal facets. Then cut the eyestalk into two longi- 

 tudinal halves with a knife, and examine with a lens under 

 water. Note the optic nerve entering the stalk, and enlarging 

 to form the optic ganglion, from which a number of bodies 

 (ommatidea} radiate outwards to the corresponding facets 

 of the cornea. The ommatidea are separated from one 

 another by pigment. Sketch. 



Examine longitudinal sections of the eye-stalks, de- 

 calcified and prepared as directed on p. 136, and note in 

 detail the above parts. Each ommatideum lies beneath 

 the corresponding corneal facet, and is made up of an outer 

 vitreous body or crystalline cone, and an inner retinula 

 formed of sensory cells and enclosing a transversely-striated, 

 spindle-shaped, refractive body or rhabdome, and closely 

 connected with the optic ganglion. Note also the pigment 

 between the ommatidea. Sketch. 



F. Structure of the Exoskeleton. 



In order to follow this out in greater detail, proceed as 

 follows : 



1. Cut through the thorax and abdomen of a crayfish 

 transversely, and note the relations of the hard and soft 

 parts. (Compare Figs. 90 and 94.) 



2. Dip a crayfish into hot water, so that the soft parts 

 come away easily. Open up the cephalothorax from the 

 dorsal side, separate some of the abdominal segments, and 



