76 



COELENTERATA 



[CH. 



further, no velum in the Scyphozoa, and there is also no nerve ring. 

 Sense organs however of an exceedingly interesting kind are present. 

 In Aurelia, for instance, there are eight minute tentacles which 

 stick out from the edge of the bell and are covered by special little 

 hoodlike outgrowths of the same (9, Fig. 33). Each of these 

 tentacles contains a hollow outgrowth from the circular canal lined 

 like it by endoderm. The endoderm cells at the tip secrete a mass 



FIG. 33. Aurelia aurita. Somewhat reduced. 



1. Mouth. 2. Circumoral processes. 3. Tentacles on the edge of the 

 umbrella. 4. One of the branching perradial canals. There are four of 

 these, and four similar interradial canals ; the perradial canals correspond 

 to the primary stomach pouches of the Hydra-tuba, the interradial 

 alternate with these. 5. One of the unbranched adradial canals. 

 8. The circular canal. 9. Marginal lappets hiding tentaculocysts. 



11. Gastral filaments, just outside these are the genital ridges. 



of calcareous particles : the ectoderm cells at the base of the tentacle 

 have produced nervous fibrillae from their bases and so the tentacle, 

 as it is caused to sway in one direction or another by the weight of 

 its heavy end, affects now some of the nerve fibrillae and now 

 others, and so produces the same effect as the stones in the ear 

 sac of a medusoid, though the construction of the Scyphozoan organ 

 is quite different. In the Trachymedusae and Narcomedusae, how- 



