CALYPTOBLASTEA LEPTOMEDUSAE 279 



Fam. Plumulariidae. The hydrothecae are sessile, and 

 arranged in a single row on the stem and branches. Nemato- 

 phores are always present. Gonophores adelocodonic. This 

 family is the largest and most widely distributed of all the 

 families of the Hydrozoa. Nutting calculates that it contains 

 more than one-fourth of all the Hydroids of the world. Over 

 300 species have been described, and more than half of these 

 are found in the West Indian and Australian regions. Repre- 

 sentatives of the family occur in abundance in depths down to 

 300 fathoms, and not unfrequently to 500 fathoms. Only a 

 few species have occasionally been found in depths of over 1000 

 fathoms. 



The presence of nematophores may be taken as the most 

 characteristic feature of the family, but similar structures are 

 also found in some species belonging to other families (p. 277). 



The family is divided into two groups of genera, the 

 Eleutheroplea and the Statoplea. In the former the nemato- 

 phores are mounted on a slender pedicel, which admits of more 

 or less movement, and in the latter the nematophores are sessile. 

 The genera Plumularia and Antennularia belong to the Eleu- 

 theroplea. The former is a very large genus, with several 

 common British species, distinguished by the terminal branches 

 being pinnately disposed, and the latter, represented by A. 

 antennina and A. ramosa on the British coast, is distinguished 

 by the terminal branches being arranged in verticils. 



The two most important genera of the Statoplea are Aglao- 

 phenia and Cladocarpus. The former is represented by a few 

 species in European waters, the latter is only found in American 

 waters. 



Fam. Hydroceratinidae. The colony consists of a mass of 

 entwined hydrorhiza, with a skeleton in the form of anastomosing 

 chitinous tubes. Hydrothecae scattered, tubular, and sessile. 

 Nematophores present. Gonophores probably adelocodonic. 



This family was constituted for a remarkable hydroid, Clathro- 

 zoon wilsoni, described by W. B. Spencer from Victoria, 1 The 

 zooicls are sessile, and spring from more than one of the numerous 

 anastomosing tubes of the stem and branches. The whole of the 

 surface is studded with an enormous number of small and very 

 simple dactylozooids, protected by tubular nematophores. Only 



1 Trans. Boy. Soc. Victoria, 1890, p. 121. 



