PREHENSION OF FOOD. 227 



either in the form of cilia or even in tentacles. In the lowly-organized 

 rhizopods and amoebae, their soft, jelly-like body is simply applied to 

 the food, which then and there enters their body substance. The most 

 marked illustration of the mode of seizing food by means of prehensile 

 tentacles is found in the case of the hydra or polyp, — small organisms 

 whose bodies are not usually longer than one centimeter, and which, as 

 already described, are supplied with a single body cavity, around the 

 single opening of which are long, slender, retractile tentacles, themselves 

 often provided with cilia, and which are capable of grasping small sub- 

 stances which may serve as their food and conveying them to their 

 digestive sac ; the adhesive power of these tentacles is increased by a 

 number of minute spiral filaments, the so-called "urticating cysts," 

 which b3' some observers are supposed to be offensive weapons, and are 

 used to paralyze the small organisms that serve as their food. The jelly- 

 fish furnishes another example of a similar method of seizing food. 

 These prehensile tentacles may be few and simple, as in the hydra; very 

 numerous, as in the sea-anemone; and often of great length and irregular 

 form, as in the medusae. 



Bivalve mollusks, like the oyster and the clam, employ the vibration 

 of cilia for creating currents to bring the nutritive matters suspended in 

 water within their reach. When the food is solid permanent prehensile 

 organs are usually present, though they ma}' be extemporized, as in the 

 case of the amoeba, where any portion of the body surface which is 

 accidentally in contact with food may serve as a prehensile organ to 

 draw matter into the interior of its body. In a higher stage of develop- 

 ment we find that tentacles are absent, but that their function is assumed 

 by flexible portions of their body, commonly called arms, which are 

 provided with a number of minute adhesive organs, which serve to seize 

 their food, and whose flexibility enables them to convey it to their oral 

 aperture. This form of prehension of food is seen in the star-fish. In 

 the sea-urchin a considerable advance is seen in the method of prehension 

 of food. The mouth itself is there the prehensive organ, and is provided 

 with five sharp teeth, each standing in a single jaw, and capable of being 

 projected so as to seize as well as masticate the prey. Univalve mollusks, 

 such as the snail, have again another organ, the tongue serving as an 

 organ of prehension. In this case the tongue is long and covered with 

 minute recurved teeth or spines, by which the food is seized and drawn 

 into the mouth, the upper part of which is armed with a sharp, horny 

 plate. In the cuttle-fish, again, the organs of prehension of food have 

 advanced still further in their development. The tongue is still present 

 as a prehensile organ; the jaws, represented by a pair of hard mandibles 

 like the beak of the parrot, and working vertically ; and in addition to 

 these several powerful prehensile tentacles, provided with powerful 



