MICROSCOPIC FAUNA. 169 



Genus Hydra^ of this there are four species. 



1. H. viridis. 3. H. communis or vulgaris. 



2. H.fusca. 4. H. atte7iuata. 



I have never found H. viridis or atte7iuata in the canal, but 

 H. fusca and H. communis are very common. The genus comes 

 from the tribe Ccelenterata, which is divided into two classes — 



(i) Nematophora, (2) Hydrowa. 

 This last has two sub-classes — 



(i) ScyphomeduscB. (2) Bydromedusce. 



There are six orders belonging to this last sub-class, and under 

 the first of these orders, Gymnoblastea, there are ten families, of 

 which one is Hydride?, and under this again is the genus Hydra. 



In this part of the country I have found H. viridis only in 

 one pond, which is in the middle of the garden of Charlcombe 

 Grove. Specimens have also been brought to me from a pond in 

 Dyrham Park, but I have never found any in the canal. Neither 

 have I found there H. atteuuafa, and I see in the " Micro. Dic- 

 tionary " that this species is said to be found in ponds and to be 

 rare. 



The distinctive characteristics of these species appear to be 

 as follows : — H. viridis. — Leaf-green in colour, body cylindrical 

 or slightly tapering towards base ; tentacles, six to ten, shorter 

 than the body, narrowest at margin. 



H. fusca. — Body brown or greyish, lower half suddenly attenu- 

 ated; tentacles, six to eight, several times longer than the body. 



H. vulgaris. — Body orange-brown, yellowish, or reddish, 

 cylindrical ; tentacles, seven to twelve, as long or longer than the 

 body, tapering. 



H. atfenuata. — Body pale olive green, attenuated below ; 

 tentacles pale, considerably longer than the body, six to ten in 

 number. 



It will be perceived that the principal distinguishing marks of 

 the various species are the colour and the length of the tentacles. 

 The habits and colours of all the species are the same. 



If a Hydra is caught and kept well fed and in a healthy con- 

 dition, it will present many points of curious and unexpected 

 interest. The entire mass of the body and tentacles is formed of 

 sarcode, and on the surface it is frequently irregularly roundish or 



Journal of Microscopy and Natural Science. 



New Series. Vol. III. 1890. n 



