2G NATURAL HISTORY OP 



hollow, but in some other species the body cavity is 

 obliterated, and they only show the ectodermal and 

 endodermal layers before described. In many of the 

 hydroids the tentacles are not arranged in a regular 

 row round the mouth, but arise in one or more circles 

 which sometimes spring from various parts of the 

 body. 



The object of the tentacles is to seize the prey upon 

 which the hydra feeds, and to convey it to the mouth, 

 but no farther.* The mouth is slowly stretched over 

 the food until it is all received into the body cavity, 

 where it is passed round and round until what is 

 nutrient has been absorbed, after which the remainder 

 is ejected through the oral orifice. It is strongly in- 

 sisted by many observers that the endoderm cells have 

 active amoeboid movements, sending out pseudopodia 

 during digestion to such an extent as to nearly or quite 

 obliterate the digestive cavity. Mr. Parker, in the 

 communication before referred to, considers that solid 

 food particles actually pass into the cells, so that the 

 hydra presents the characteristically protozoan mode 

 of digestion. In one case he observed a diatom frus- 

 tule embedded in the protoplasm of a cell. This intra- 

 cellular digestion is regarded by Prof. Metschnikolf f 

 as the rule in most of the true Coelenterata, having 

 been observed in all the chief groups of that sub- 

 kingdom. 



The hydras, unlike the rest of the hydroids, are 

 capable of locomotion either by free swimming through 

 the water, or by crawling with a looped motion like 

 that of a caterpillar. 



* Mr. Hertwig, Quar. Journ. of Mic. Soc. xx. (1880), 243. 

 t Zool. Anzeig. iii. (1880), 261. 



