PHYLUM CCELENTERATA 



97 



The mouth (Figs. 41, C, D, and 43, mth) leads into an 

 enteric cavity which occupies the whole interior of the 

 manubrium, and from its dilated base sends off four delicate 

 tubes, the radial canals (rail, c), which pass at equal 

 distances from each other through the substance of the 

 umbrella to its margin, where they all open into a circular 

 canal (cir. c), running parallel with and close to the 

 margin. By means of this system of canals the food, taken 

 end lam 



Fig. 43. — Dissection of a medusa with rather more than one-quarter of the umbrella 

 and manubrium cut away (diagrammatic). The ectoderm is dotted, the endo- 

 derm striated, and the mesoglcea black, cir. c, circular canal; end. lam, 

 endoderm lamella; gon, gonad; /, lithocyst; tnnb, manubrium; mth, mouth; 

 rad. c, radial canal; vl, velum. 



in at the mouth and digested in the manubrium, is dis- 

 tributed to the entire medusa. 



The edge of the umbrella is produced into a very narrow 

 fold or shelf, the velum (Fig. 43, vl), and gives off the 

 tentacles (/), which are sixteen in number in the newly- born 

 medusa (Fig. 4t, D), but which are very numerous in the 

 adult. At the bases of eight of the tentacles — two in each 

 quadrant — are minute globular sacs (/), each containing a 



