A MANUAL OF ZOOLOGY 13 



It wijl be necessary to study well prepared cross sec- 

 tions of the rays in order to demonstrate the nerve aad the 

 blood vessels that run along with the water vascular 

 system. Make ajxawin.gLot_a_croM_section_of_aj:a Note 

 the eyes in the ends of the rays. Examine for size, shape, 

 and color. Are the eyes protected in any way? At the 

 base of some of the spines branched jaw-like projections, 

 the pedicellarlae, may be found. These are best seen on 

 sections. Study the skeleton already prepared. Are all 

 the ossicles arranged alike? Cut across one of the rays. 

 The ambulacral ossicles are found forming the roof of the 

 ambulacral groove. What is the relation of the ambulacral 

 pores to theossieles? A single row of ossicles just outside 

 of these form the Inter-ambulacral ossicles. Note that these 

 ossicles l)ear movable spines. In the middle of the floor 

 of the ray will be found the vertebral ridge. Of what is 

 it formed? Remove the disk. The mouth opening will 

 appear. Note the partitions between the rays, inter-radial 

 partitions. ^ In these find the reproductive orifice. Treat 

 some of the skeleton With dilute HCl' and note results. 

 From the effect of the HCl, what is the chemical composi- 

 tion of the ossicles? By means of a pair of scissors cut 

 a slit from the tip of the rays of the trivlum along tlie 

 sides of the dorsal surface. This must be done with 

 care so as not to injure the organs underneath. Turn back 

 the cut portion to the disk. When this is done the diges- 

 tive system can be seen. Note a thin membrane, the mes- 

 entery, which holds the large greenish bodies (hepatic 

 coeca) to the dorsal surface as it is turned back. What is 

 the size, the arrangement, and the attachment of the he- 

 patic coeca? Demonstrate the tube or tubes, that lead 

 from it to the stomach immediately under the disk. Note 

 the two portions of the stomach: the central cavity; the 

 cardiac portion; and the pyloric portion or pouches, that 

 extend into each of the rays slightly. From the structure 

 of the mouth and stomach whiat would' be the conclusion 

 as to the kinds of food adai)ted to the starflsht In some 

 specimens the cardiac portion may be fpi|nd protruding 

 from the mouth.' Whatdbes this indicate? What is the size 

 and extent of the oesophagus? The retractor and "the pro- 

 tractor muscles of the stomach will be seen as white shin- 



