2 8o COELENTERATA HYDROZOA chap. 



a few specimens have hitherto been obtained, the largest being 

 1 inches in height by 4 inches in width. In general appearance 

 it has some resemblance to a dark coloured fan-shaped Oorgonia. 



Fam. Campanulariidae. — The hydrothecae in this family are 

 pedunculate, and the gonophores adelocodonic. 



In the cosmopolitan genus Camixmularia the stem is mono- 

 siphonic, and the hydrothecae bell-shaped. Several species of this 

 genus are very common in the rock pools of our coast between tide 

 marks. Halecium is characterised by the rudimentary character 

 of its hydrothecae, which are incapable of receiving the zooids even 

 in their maximum condition of retraction. The genus Lafoea is 

 remarkable for the development of a large number of tightly packed 

 gonothecae on the hydrorhiza, each of which contains a blasto- 

 style, bearing a single gonophore and, in the female, a single 

 ovum. This group of gonothecae was regarded as a distinct 

 genus of Hydroids, and was named Coppinia} Lafoea dumosa 

 with gonothecae of the type described as Coppinia arcta occurs 

 on the British coast. 



PerisipJwnia is an interesting genus from deep water off the 

 Azores, Australia, and New Zealand, with a stem composed of 

 many distinct tubes. 



The genus Zygophylax, from 500 fathoms off the Cape Verde, 

 is of considerable interest in having a nematophore on each side 

 of the hydrotheca. According to Quelch it should be placed in 

 a distinct family. 



Ophiodes has long and very active defensive zooids, protected 

 by nematophores. It is found in the Laminarian zone on the 

 English coast. 



Fam. Eucopidae. — The hydrosome stage of this family is 

 very similar to that of the Campanulariidae, but the gonophores 

 are free-swimming Medusae of the Leptomedusan type. 



One of the best -known genera is Ohelia, of which several 

 species are among the commonest Hydroids of the British 

 coast. 



Clytia johnstoni is also a very common Hydroid, growing on 

 red algae or leaves of the weed Zostera. It consists of a number 

 of upright, simple, or slightly branched stems springing from a 

 creeping hydrorhiza. When liberated the Medusae are globular 

 in form, with four radial canals and four marginal tentacles, but 

 ' See C. C. Nutting, Prac. U.S. National Museum, xxi. 1899, p. 747. 



