VI PRACTICAL DIRECTIONS 379 



the basal segment of the antennule with scissors, so as to expose the 

 auditory sac. Cut this out and place it on a slide, carefully removing 

 the muscles surrounding it, as well as the setse around its aperture. 

 Note the ooritained grains of sand (otoliths)., 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 auditory 

 setts of various sizes, arranged in rows, and that branches of the 

 antennulary nerve run up the stem of each seta. Sketch. 



4. The Eyes. Rentove one of the eye-stalks, and note the apparently 

 black, uncajcified, oval portion of the cuticle {cornea) at its distal end. 

 Strip this off, ar(d 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 out the eye^stalk into two longitudinal 

 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. 



3. Examine longitudinal sections of the eye-stalks, decalcified 

 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 con- 

 nected 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 ;— 



I. Cut through the thorax and abdomen of a crayfish transversely, 

 and note the relations of the hard and soft parts. (Compare Figs. 83 

 and 87). 



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 clean thoroughly. Examine 

 the joints and make out the relations of the endophragmal system 

 (p. 377), looking something like a lattice-girder. 



3. Examine your preparation of the eye once more under the micro- 

 scope and notice the part where the section passes through the Qiiter 



