ARBACIA PUNCTULATA 199 



epineural cavities. The hyponeural or motor system is consider- 

 ably reduced and consists of five very short radial nerves with 

 still shorter five transverse pieces representing the broken up 

 ring. The entoneural system is inclosed in the aboral sinus. It 

 consists of an aboral pentagon or ring with five genital nerves. 



Reproductive system. The reproductive organs are similar 

 in both sexes. They consist of five simple, interradial gonads 

 which open on the genital plates. Organs of copulation are 

 absent. The reproductive cells are emptied into the water. 

 The development is characterized by the formation of a larva 

 known as pluteus. 



Instructions 



1. Place a sea-urchin in a dissecting pan with water. Ex- 

 amine the spines and remove one of each of the three types of 

 spines: a long, sharp-pointed equatorial spine, a short spine with 

 the broadened flat end and an intermediate spine with blunt end. 

 Study their surface under dissecting microscope. Make a draw- 

 ing showing the three spines twice their natural size. 



2. Remove all spines taking care not to injure the ambulacral 

 tubes. Place the sea-urchin with its oral surface uppermost. 

 Make a drawing twice natural size showing the five teeth, cir- 

 cular lip, peristomial membrane, five pairs of oral papillae, five 

 pairs of branchiae, five clusters of ambulacral feet, spine tubercles 

 and the pedicellariae of the peristomial membrane. Label also 

 radii and interradii. 



3. Remove with the aid of a forceps an ambulacral foot and 

 examine it under dissecting microscope. Make a drawing 

 showing the foot and its sucker. 



4. Remove an entire cluster of ambulacral feet and examine 

 the place under dissecting microscope. Make a drawing showing 

 the pairs of ambulacral pores and the minute sphaeridium. 



5. Remove a pedicellaria, place it in glycerine on a slide under 

 a cover glass, examine under microscope and make a drawing 



