64 INVERTEBRATE ANIMALS. 
warts or tubercles, and of being extended to a 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 thaneight inches.) The tentacles are the organs by means 
of which the Hydra obtains its food, consisting chiefly of minute 
aquatic organisms, such as small worms, insects, Crustacea and 
Rotifera. These are seized by the tentacles and gradually 
drawn into the mouth; but in addition to this merely mechan- 
ical action, the tentacles appear 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 capa- 
cious 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. 16, a, 4) 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 as- 
serted to be present. There are no internal organs of any kind. 
Physiologically, therefore, the Yydra presents little advance upon 
the higher Protozoa, such as the /ufusoria. There is a per- 
manent mouth, surrounded by permanent and special organs 
adapted for the seizure of food. There is also a permanent in- 
ternal cavity for the reception and digestion of the food, but this 
is not shut off 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 essen- 
tially 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 annihila- 
tion. 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 naturalists, discovered that the Hydra could be 
mechanically divided with a knife into any number of frag- 
ments, 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, 
