1 86 MANUAL OF ELEMENTARY ZOOLOGY 



oblong gullet are two grooves, the siphonoglyphs, down 

 one of which and up the other currents of water flow even 

 when the rest of the mouth is closed. As in Hydra, the 

 tentacles are hollow and there is no medusa, though a 

 planula larva is found. Corals are related to anemones, 

 but secrete a calcareous shell or skeleton on the surface of 

 the ectoderm of the lower part of the body. Thus the 



Fig. iiS. — A small specimen of the Common Jelly-fish. 

 Aurelia aitriht, natural size. 



Note the horseshoe-shaped gonads, showing through the transparent tissues ; the 

 radial canals, alternately branched anil unliranched : the little sense tentacles in 

 notches each opposite the middle branch of a canal ; the marginal tentacles ; 

 and the arms of the manubrium, each folded ami fringed. 



" coral insect " is a polyp. The large jelly-fish (Aca/ep/nc) 

 differ from the little medusae of the hydroids in having no 

 velum or nerve ring, and developing their generative cells 

 on the endoderm of the stomach wall ; their radial canals 

 are often branched. Under the four generative organs lie 

 sul'gemla/ pits of the subumbrella ectoderm. When "there is 

 a polyp generation (Hydra tuba), it has hollow tentacles, four 



