DIVISIONS OF THE HYDROZOA. 59 



length which is in some species many times longer than the 

 body itself. (In Hydra fusca the tentacles can be protruded 

 to a length of more than eight inches.) The tentacles are the 

 organs by means of which the Hydra obtains its food, con- 

 sisting chiefly of minute aquatic organisms, such as small 

 worms, insects, Crustacea and Motifera. These are seized 

 by the tentacles and gradually drawn into the mouth ; but in 

 addition to this merely mechanical action, the tentacles ap- 

 pear to exercise a benumbing or even fatal influence upon the 

 animals grasped by them — this being apparently due to the 

 thread-cells with which they are furnished. The mouth in 

 the Hydra opens directly into a capacious cylindrical cavity, 

 which is excavated along the whole length of the body, and 

 which is both the body-cavity and the stomach in one. This 

 cavity (Fig. 13, a, b) is filled with water derived from the 

 exterior, and also with the nutritive particles derived from 

 the food. Indigestible fragments appear to be rejected by 

 the mouth, though an anal aperture has been asserted to be 

 present. There are no internal organs of any kind. Physio- 

 logically, therefore, the Hydra presents little advance upon 

 the higher Protozoa, such as the Infusoria. There is a per- 

 manent mouth, surrounded by permanent and special organs 

 adapted for the seizure of food. There is also a permanent 

 internal cavity for the reception and digestion of the food, 

 but this is not shut oflE' from the general cavity of the body. 

 There is no organ for the propulsion of the nutritive fluid 

 through the body, no nervous system or organs of sense, and 

 no special respiratory or excretory organs. Another and 

 striking proof of the essentially low position of the Hydra in 

 the animal scale is to be found in its extraordinary capacity 

 of resisting mutilation, or, in fact, mechanical injury of any 

 kind short of absolute annihilation. The briefest illustration 

 of this fact is all that can here be given, but with that the 

 name of Trembley of Geneva must be associated. This well- 

 known observer, in a long series of experiments, most of 

 which have been successfully repeated by subsequent nat- 

 uralists, discovered that the Hydra could be mechanically 

 divided with a knife into any number of fragments, with the 

 sole result that each and all of these possessed the power 

 of developing themselves into fresh and independent polypites. 

 Further, the animal could even be turned inside out, with 

 a necessary transposition of the ectoderm and endoderm, 

 without any apparent inconvenience or interference with its 

 health. 



