X GYMNOBLASTEA ANTHOMEDUSAE 269 



parasiticum is said to be a parasite living at the expense of 

 Uudendrium racemosum. Mnestra is a little Medusa which 

 attaches itself by its manubrium to the Mollusc Phyllirhoe, and 

 may possibly feed upon the skin or secretions of its host. 



Nearly all the species of the order are found in shallow sea 

 water. Stylactis vermicola and the " Challenger" specimen of Mono- 

 eaulus imperator occur at a depth of 2900 fathoms, and some 

 species of the genera Uudendrium and Myriothela descend in 

 some localities to a depth of a few hundred fathoms. Gordylo- 

 phora is the only genus known to occur in fresh water. From 

 its habit of attaching itself to wooden piers and probably to 

 the bottom of barges, and from its occurrence in navigable rivers 

 and canals, it has been suggested that Cordylophora is but a 

 recent immigrant into our fresh -water system. It has been 

 found in England in the Victoria docks of London, in the Norfolk 

 Broads, and in the Bridgewater Canal. It has ascended the 

 Seine in France, and may now be found in the ponds of the 

 Jardin des Plantes at Paris. It also occurs in the Elbe and 

 in some of the rivers of Denmark. 



The classification of the Gymnoblastea is not yet on a satis- 

 factory basis. At present the hydrosome stage of some genera 

 alone has been described, of others the free-swimming Medusa 

 only is known. Until the full life-history of any one genus has 

 been ascertained its position in the families mentioned below may 

 be regarded as only provisional. The principal families are : — 



Fam. Bougainvilliidae. — The zooids of the hydrosome have a 

 single circlet of filiform tentacles at the base of the hypostome. 

 In Bougainvillia belonging to this family the gonophores are 

 liberated in the form of free-swimming Medusae formerly known 

 by the generic name Hippocrene. In the fully grown Medusa 

 there are numerous tentacles arranged in clusters opposite the 

 terminations of the four radial canals. There are usually in 

 addition tentacular processes (labial tentacles) on the lips of 

 the manubrium. Bougainvillia is a common British zoophyte 

 of branching habit, found in shallow water all round the coast. 

 The medusome of Bougainvillia ramosa is said to be the common 

 little medusa Margelis ramosa} Like most of the Hydroids it 

 has a wide geographical distribution. Other genera are Peri- 

 gonimus, which has a Medusa with only two tentacles; and 

 1 Hartlaub, Wiss. Meeresunt. deutsch. Meere in KielT^.Y.I. 1894, p. 1. 



