METAZOA COELENTERA. 107 



mouth. Fig. n is a so called scyphostoma with four 

 tentacles extended, surrounding a mouth at the end of an 

 oral cone. 



It is interesting to note that the scyphostoma of Cyanea 

 has a horny sheath similar to that possessed by the Tubu- 

 lariae. No such sheath has been observed in Aurelia. 

 One feature of the anatomy of the scyphostoma is of im- 

 portance. Four ridges extend from the inner wall into 

 the central cavity and divide its outer portion into four 

 chambers. These are probably the beginnings of the 

 mesenteries of the Anthozoa. 



The scyphostoma of Aurelia (fig. 12) normally develops 

 sixteen tentacles. It is converted into the so called stro- 

 bila in the following way. A horizontal constriction takes 

 place just below the outer base of the tentacles (fig. 13) ; 

 this is followed by another (fig. 14), and a third (fig. 15), 

 and still others, until nearly the whole body is divided 

 into saucer-like sections. At the base of these sections or 

 discs (fig. 1 6) is developed a circle -of tentacles similar to 

 those at the top of the scyphostoma when it began to di- 

 vide. Below this circle of secondary feelers is the rem- 

 nant of the old scyphostoma. The strobila stage succeed- 

 ing the scyphostoma is now completed. The uppermost 

 and oldest disc which has become eight lobed, separates 

 first, then the others follow in succession, fig. 17 rep- 

 resenting the last disc about to drop off. This free stage 

 of the Discophoran is known as the ephyra. After sepa- 

 ration the ephyra turns over (fig. 18, enlarged ten diame- 

 ters, with the lips of the mouth prominent) and is a well 

 developed medusa (fig. 19, natural size). The parts are 

 better seen in the larger species, A. aurita Linn. (Nos. 135, 

 136). The most important organ of the adult is the 

 umbrella ; this is usually divided into eight lobes (No. 

 136). Hanging from the center of the lower side are four 

 long oral appendages. A system of tubes extends from 

 the central digestive cavity to the circumference, and alter- 

 nates with another system which connects with the large 

 genital organs on the dorsal side of the Aurelia. 



