178 THE FLORAL WORLD AND GARDEN GUIDE. 
haying a minute opening at the apex, others closed. The use of the 
perforated villi appears to be that of conveying the nutritive matter 
of the ingested food into the interior and nourishment of the 
animal. 
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Hydra fusca, containing the larva of an insect partially digested, magnified. 
(After Corda.)—a, superficial cells of the integument ; 6, muscular stratum, 
formed of minute granular coloured cells; ¢, villous tunic; d, anus; e¢, 
sucker. 
There are four distinct species of Hydra. The commonest is 
H. viridis, common in still waters, and commonly adherent to the 
duckweed. It is of a leaf-green colour, and has six to ten tenta- 
cles, which are shorter than the body. H. vulgaris occurs in 
slowly running waters and weedy ponds. It is orange brown, or 
yellow, with seven to twelve tentacles, which are as long as the 
body or longer. H. attenuata is a rare pond species, with pale 
olive-green body, and pale tentacles, which are longer than the body. 
H. fusca is also rare, but particularly worth finding, as it has been 
so frequently the subject of experiment. It has a brown or greyish 
