HYDRACTINIA. 33 



because he discovered the genus early in 1839, while Mr. 

 Hassall did not define Echinochorium until November, 1840. 



The polypes in Hydractinia are gregarious or clustered, 

 originating from a horny crust that spreads over the foreign 

 body to which they affix themselves. This character is un- 

 doubtedly the principal one that distinguishes the genus from 

 Clava. The tentacula, although in a single whorl, make 

 an approach to become biserial, for every alternate one 

 stands forward a little, and is longer than the one exterior to 

 it. They are contractile, and vary in number according to 

 the age of the individual. It is a singular fact, that, in the 

 species observed by Van Beneden, the oviparous individuals 

 had no tentacula, which apparently had dwindled away 

 from atrophy when the polype began to direct its nutritive 

 power to the development of reproductive germs. 



This peculiarity in the oviparous individuals has been also 

 remarked by M. de Quatrefages, who adds, that they are 

 smaller than the others, and destitute of an oral aperture. 

 (Ann. des Sc. Nat. xx. p. 233, 242). He therefore con- 

 cludes that they receive their nourishment from the barren 

 polypes, for, according to him, there is, underneath the 

 horny base, a network of small white opake creeping fibres 

 which anastomose freely, and have a central canal continu- 

 ous with the stomachal cavity of the polypes. (Plate I. 

 fig. 6, a.) This structure establishes and maintains a direct 

 communication between all the individuals of the same cluster 

 (lib. cit. p. 234), and renders the alimentary matter digested 

 by a single polype available to the nutrition of the entire 

 colony. \Ve are not exercising an over-caution in the ex- 

 pression of a wish that this singular part of their anatomy 

 were confirmed. Mr. Hassall has taken no notice of it ; and 

 its existence is denied by Van Beneden. (Bull, de FA cad. 

 Roy. de Brux. xii. no. 2, p. 13.) Is it not possible that, in 

 the specimen examined by M. de Quatrefages, the crust of 

 the Hydractinia had overgrown a basis previously occupied 

 by some Sertularia, and that the network he describes is the 

 remainder of the root-fibres by which that Sertularia had 

 been affixed ? 



Besides its propagation by buds and bulbules, as he hap- 

 pily names the reproductive bodies that bourgeon from the 



D 



