252 COMPARATIVE ZOOLOGY 



of the body then flows toward and collects about the 

 food, and digests it (Fig. 213). 



A higher type is seen in polyps and jellyfishes, which 

 have hollow tentacles around the entrance to the stomach 

 (Figs. 1 8, 20, 26). These tentacles are contractile, and 

 some, moreover, are covered with an immense number 

 of minute sacs, in each of which a highly elastic filament 

 is coiled up spirally (lasso cells, nettle cells). When 

 the tentacles are touched by a passing animal, they 

 seize it, and at the same mome~nt throw out their myriad 

 filaments, like so many lassos, which penetrate the skin 

 of the victim, and probably also emit a fluid, which 

 paralyzes it ; the mouth, meanwhile, expands to an ex- 

 traordinary size, and the creature is soon ingulfed in 

 the digestive bag. 



In the next stage, we find no tentacles, but the food 

 is brought to the mouth by the flexible lobes of the 

 body commonly called arms, which are covered with 

 hundreds of minute suckers ; and if the prey, as an 

 oyster, is too large to be swallowed, the stomach pro- 

 trudes, like a proboscis, and sucks it out of its shell. 

 This is seen in the starfish (Fig. 323). 



A great advance is shown by the sea urchin, whose 

 mouth is provided with five sharp teeth, set in as many 

 jaws, and capable of being projected so as to grasp, 

 as well as to masticate, its food (Figs. 48, 226). 



In mollusks having a single shell, as the snail, the 

 chief organ of prehension is a straplike tongue, covered 

 with minute recurved teeth, or spines, with which the 

 animal rasps its food, while the upper lip is armed with 

 a sharp, horny plate (Fig. 227). In many marine species, 

 as the whelk, the tongue is situated at the end of a 

 retractile proboscis, or muscular tube. In the cuttlefish, 

 we see the sudden development of an elaborate system 

 of prehensile organs. Besides a spinous tongue, it has 



