98 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



3. Between each pair of these a pair of protractor 

 muscles (Fig. 40, e) running from the upper angles of 

 each alveolus to the corresponding inter-ambulacral area. 



4. Five pairs of retractor muscles (Fig. 40, h) running 

 from the auriculae to the oral ends of the alveoli. 



5. The inter-alveolar muscles, running between the 

 faces of adjacent alveoli. 



6. Make a sketch of the lantern, with its muscles. 



d. Notice the radiating water tubes (Fig. 40, i) Avhich 

 pass out from under the outer ends of the radulae ; run 

 down over the outer surfaces of the inter-alveolar muscles, 

 and then pass out between the auriculae, and run upwards 

 along the inter-ambulacral suture to the ovarian plate. 

 Notice the flat, leaf-like ampullae upon each side of the 

 "water tube. 



€. The nervous system. As the nerve ring is situated 

 upon the inner surface of the peristome, between the 

 oesophagus and the tips of the alveoli, it is necessary to 

 carefully cut away one side of the lantern, in order to 

 expose it. This may be done by breaking the alveoli 

 away in small pieces, with a pair of strong scissors. After 

 exposing the nervous system, notice : — 



1. The circum-oral nerve ring, a pentagonal ridge 

 around the oesophagus, just inside the tips of the five 

 teeth. 



2. The five radiating nerve fibres running along the 

 ambulacral sutures from the angles of the pentagon to the 

 ovarian plates outside the water tubes, or between them 

 and the corona. 



