LOBSTEE OE CEATFISH 151 



pendages' as on the body ? How is it modified 

 at the joints ? Does it bear any outgrowths ? 

 If so, where are they and what is their structure ? 

 Put a piece of the exoskeleton into boiling water 

 for a few minutes. What changes take place in 

 it ? Put a piece into dilute hydrochloric acid, 

 consisting of one part acid to four parts water. 

 "What happens? Explain. 



F. The organs of special sense. 



a. The tactile organs — i. e., the antennaj, the anten- 

 nules, and the palpi on the oral appendages — 

 have already been studied. 



h. The eye. — Eemove one of the eye stalks, note again 

 its gross structure, then examine with a lens. 

 ITotice its transparent outer covering, the cor- 

 nea. "What is its shape? How is the surface 

 marked ? Each area or facet corresponds to one 

 of the elements of which the compound eye is 

 composed. What is their shape ? Arrangement? 

 Estimate the number of facets. Make a longi- 

 tudinal section of the eye and its stalk, cutting 

 the hardened portion with scissors, the rest with 

 a scalpel. Examine with a strong lens or the 

 low power. Note the edge of the cornea; the 

 ring of opaque exoskeleton to which it is con- 

 nected by a membrane ; the striated mass marked 

 by radiating lines, indicating the striated spin- 

 dles and the crystalline cones; the dark-col- 

 ored central mass ; the optic ganglion, from 

 which runs through the axis of the stalk; the 

 optic nerve, and the muscles lying on each 

 side of the optic nerve. Examine as closely as 



