NATURAL HISTORY. 



The little Velella, of the sub-order Discoida, has been compared to a little raft with an obliquely 

 placed upright sail ; the raft has its system of canals, and the thin membranous sail is the air-sac. It 



is cartilaginous, and the concentric tubes 

 found within open externally. Below the 

 disc are the nutritive and generative zooids, 

 and there is usually a large polype in the 

 midst of this crowded submarine colony. 

 There are tentacles on the edge of the disc, 

 which may be bright blue, purple, or brown 

 in colour. It sails along with its upright 

 membranous part, and is kept up by the 

 air canals. The generative zooids produce 

 medusae, which become free. L. Agassiz 

 describes the medusa of Velella tiiutica as a 

 long bell, with a short proboscis in the 

 upper part of the cavity, which is connected 

 with the outside by a tubular opening. 

 Eggs are the product of the medusae, and 

 they develop into Velellse. 



ORDER HYDROIDA. 



The fresh-water polype is a common 

 name for several species of the genus Hydra, 

 which are to be found in ponds and slow 

 streams, hanging to the under surface of 

 floating leaves and upon the stems of water 

 plants. If in the summer time a glass jar 

 is tilled with clear pond water, and some of 

 ^ Q <j uc k- W eed also, minute bodies, like 



pieces of green sewing silk, about the sixth of an inch long and very slender, will be seen on the sides 



of the vessel, or on the weed, beneath the water. On using a low magnifying power, the little 



object is seen to be fixed by a 



small suckei'-like base, and to 



have a cylindrical body, ter- 



minating in a crown of feelers, 



or tentacles, six to ten in 



number, and shorter than the 



body (Hydra viridis). 



It usually hangs down- 



wards, and the tentacles stretch 



out, curve, expand, and contract, 



whilst the body elongates, and 



often, on a slight alarm, con- 



tracts, and becomes more or 



less globular in shape. A 



minute crustacean swims along 



close to the Hydra, and one of 



the tentacles touches it. The 



movement of the living prey is 



arrested at once, the tentacle adheres to it, and then the whole crown of feelers comes to help, 



and the morsel is dragged to the mouth and slowly passes into the body. There, enclosed in 



the visceral cavity, the victim is slowly digested, and the undigested matter is, after a time, 



returned by the mouth. Tired of its position, the Hydra may be seen to bring its crown of tentacles 



VELELLA LIKBOSA. 



