144 PHYLUM C@LENTERA. 
etc.) are often seen wandering to and fro on the surface 
of the Hydra, but these wonted visitors do not provoke the 
stinging cells to action. 
So simple is Hydra, that a cut-off fragment may grow 
into an entire animal. Thus the Hydra may be multiplied 
by being cut in pieces. The two conditions of a fragment 
regenerating a whole are—(1) that the fragment be not too 
small, and (2) that it bea fair sample of the various kinds 
of cells in the body. Thus neither a little corner off the 
base nor the tip of a tentacle will grow into a new Aydra. 
If the animal be turned inside out (a delicate operation), 
the status guo is soon restored. The Abbé Trembley, who 
first made this experiment, thought that the out-turned 
endoderm assumed the characters of the ectoderm, and 
that the inturned ectoderm assumed the characters of 
endoderm. But this is not the case. Either the animal 
rapidly rights itself by turning outside in, or,. if this be 
prevented, the inturned ectoderm disappears internally, 
and by growing over the out-turned endoderm, from the 
lips downwards, restores the normal state. 
In favourable nutritive conditions, the Hydra forms buds, 
and on these a second generation of buds may be developed. 
A check to nutrition or some other influence causes the 
buds to be set ‘adrift. Sometimes a Aydra divides across 
the middle, and each half grows into a complete polyp in 
afew days. Besides these asexual modes of multiplication, 
the usual sexual reproduction occurs. ; 
General structure.—The tubular body consists of two 
layers of cells, ze. the animal is diploblastic. The cavity 
is the gut, and it is continued into the hollow tentacles. 
These, when fully extended, may be much longer than the 
body. The mouth is slightly raised on a disc or hypostome. 
Of the two layers of cells, the outer or ectoderm is trans- 
parent, the inner or endoderm usually contains abundant 
pigment. On the tentacles especially, even with low power, 
one can see numerous clumps of clear stinging cells. The 
male organs appear as ectodermic protuberances a short 
distance below the bases of the tentacles; the ovary, with 
a single ovum, is a larger bulging farther down. Both male 
and female organs may occur on the same animal, either 
at one time or at different times, but often they occur on 
