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10, shorter than the body, which is cylindrical in shape, narrow- 
ing downwards . The tentacles are frequently beset with a small 
infusorial animalcula ( Trichodina pediculus ) , which runs up and 
down them with extreme vivacity. This species is pretty 
common, though, on account of its small size, it requires close 
inspection to discover it. 
H. vulgaris ; about the same size as H. viridis, of a light- 
brown colour usually, but sometimes red. The colour depends 
probably on the nature of the animaFs food. Tentacles 6 to 
12, about the length of the body; in still ponds or slowly 
running water, common. 
H.fusca. This is the most beautiful of all the species; 
though said to be rare in England, I find it tolerably abundant 
in weedy ponds and ditches. It is this species in which an 
intestinal canal has been affirmed to exist ; tentacles 6 to 9, 
capable of extraordinary extension, fifteen or twenty times the 
length of its body; colour, pale brown ; when it has been long 
without food, it is almost white. I have often noticed the 
body of this species covered with numerous white spots, which 
appears to be indicative of disease. 
H. attenuata. I know nothing of this species : it is described 
as being of a light oil-green colour, with paler tentacula, longer 
than itself ; body larger than H. vulgaris and of a more graceful 
form. The tentacles hang like silken threads in the water, 
waving to and fro without assuming the regular circular posi- 
tion which they do in H. viridis. 
The books of Trembley and Baker abound with various 
experiments practised on the bodies of hydrse; and the 
Memoir of the first-named observer contains many admirably 
executed figures. The results of these experiments may 
be summed up in the language of Dr. G. Johnston : — 
“ If the body is halved in any direction, each half in a short 
time grows up a perfect hydra ; if it is cut into four or eight, 
or even minced into forty pieces, each continues alive and 
develops a new animal, which is itself capable of being multi- 
plied in the same extraordinary manner. If the section is 
made lengthways, so as to divide the body into two or more 
slips, connected merely by the tail, they are speedily resoldered, 
like some heroes of fairy tale, into one perfect whole; or if 
the pieces are kept asunder, each will become a polype, and 
thus we may have two or several polypes with only one tail 
between them ; but if the sections be made in the contrary 
direction — from the tail towards the tentacula — you produce 
a monster with two or more bodies and one head. If the 
tentacula — the organs by which they take their prey, and on 
which their existence might seem to depend — are cut away, 
they are reproduced, and the lopped off parts remain not long 
