i) 
“NI 
REPORT ON THE HYDROIDA. 
Acanthella, nov. gen. 
Name, a diminutive noun formed from dkava, a thorn, in allusion to the spine-bearing 
terminations of the branches. 
Generic Cuaracter. Trophosome.—Hydrocladia pinnately disposed ; hydrocladia- 
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 Plumularide the genus Acanthocladiwm 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 hydrocladia. 
No part of the gonosome was present in the specimens of the only species referrible to 
Acanthella. 
Acanthella effusa, Busk, sp. (Pl. VL). 
Plumularia effusa, Busk, Voyage of the “ Rattlesnake,” 1852, vol. 1. p. 400. 
Trophosome.—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. Hydrothecee 
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 hydrotheca, lateral 
nematophores springing from points close to its margin ; hydrocladial imternodes 
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 obvious 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 hydrothecz, but are directly confluent by their base with the iter- 
nodes which carry them. In the angle between these more distal spines and the sup- 
