REPORT ON THE HYDROIDA. 27 



Acanthella, nov. gen. 



Name, a diminutivo noun formed from a.Kav6a, a thorn, in allusion to tlie spine-bearincr 

 terminations of the branches. 



Generic Character. Tro^fhosome. — Hydrocladia pinnately disposed ; liydrocladia- 

 bearing branches terminating in simple jointed prolongations in which the places of 

 the hydrocladia are taken by spine-like appendages. 



Gonosome not known. 



The genus Acanthella, so far as regards its trophosome, represents among the Eleu 

 theroplean section of the Plumularidse the genus Acanthocladium of the Statoplea. The 

 peculiar terminations of the branches are essentially the same in both, and the lateral 

 spines which these support are in both cases the morphological equivalents of hydi-ocladia. 

 No part of the gonosome was present in the specimens of the only sj)ecies referrible to 

 Acanthella. 



Acanthella effusa, Busk, sp. (PI. VI.). 



Plumularia effusa. Busk, Voyage of the "Rattlesnake," 1852, vol. i. p. 400. 



Trojjhosome. — Colony attaining a height of twelve inches ; main stem springing from 

 a dense mass of entangled filaments, monosiphonic, giving off a multitude of- scattered 

 subdivided branches, which carry the hydrocladia, every subdivision ending in a spine- 

 like continuation which is composed of numerous internodes, each internode supporting 

 two or more stout blunt spines ; hydrocladia one-tenth of an inch in length. Hydrothecse 

 pitcher-shaped, with entire margin, adnate by their whole height to the rachis ; mesial 

 nematophore single, springing from a point close to the base of the hydi-otheca, lateral 

 nematophores springing from points close to its margin ; hydi'ocladial internodes 

 separated from one another by a very well-marked joint, and each carrying a hydrotheca. 



Gonosome not known. 



Acanthella effusa is a remarkable and beautiful species, and, like Acanthocladium 

 affords in the curious terminations of its branches an example of the extent to which 

 the hydrocladia may be modified, and yet allow of the recognition of their homological 

 significance. 



A very olwious transition may be traced from the simple spines, which occur towards 

 the distal extremity of the branch, backwards into the true hydrotheca-bearing ramuli. 

 In fact some of the posterior or more proximal spines still carry near the base a single 

 hydrotheca, with its mesial and lateral nematophores. These spines are also borne on the 

 summit of a thick process from the internode, while the more distal spines are not only 

 quite destitute of hydro thecse, but are directly confluent by their base with the inter- 

 nodes which carry them. In the angle between these more distal spines and the sup- 



