AND PHYSIOLOGY OF THE SPONGIAD^. 
1107 
very distinctly to be seen, but in other species this structure is totally wanting. It 
therefore ceases to be of value as a generic character, and becomes a specific one only. 
Under these circumstances I propose the following modification of the previously pub- 
lished generic characters : — 
Sponge massive, suborbicular. Skeleton consisting of fasciculi of spicula. Fasciculi 
radiating from a basal or excentrical point to the surface. Intermarginal cavities 
unsymmetrical, confluent. Propagation by internal or external gemmulation. 
This genus afibrds us one of the few instances in which we may avail ourselves of 
external form as a generic character ; but even in Tethea we approach exceptions to the 
rule in the depressed form of T. Collingsii, Bowerbank, MS., as exhibited in the only 
perfect specimen of that species which I have seen, and in the still more depressed form 
of T. spinularia, Bowerbank, MS. 
Although the skeleton-structures in the species of this genus differ to an exceedingly 
slight extent, the subsidiary spicula vary exceedingly in the different species. In some, 
temate spicula are numerous, and in others they are entirely absent ; and stellate forms 
of spicula occur in many varieties of form. 
The sponges of this genus appear to be highly organized. Audouin and Milne- 
Edwaeds saw the oscula open and the excurrent streams in action, and I have seen the 
same myself in a specimen of T. Lyncurium. My friend Mr. George Clifton, of Free- 
mantle. Western Australia, in a letter dated 25th January, 1861, writes, “I have sent 
you several fine specimens of Tethea. When these animals are first taken out of the 
water they are of a brilliant orange-colour, and commence squirting water from the oscula 
situated on the centre of the upper surface ; they also contract considerably, but on being 
replaced in their native element they regain their natural size and reabsorb water.” 
The mode of propagation varies in different species. In T. cranium and T. similli- 
mus, Bowerbank, MS., it is by internal gemmulation, in T. Lyncurium by external gem- 
mulation ; and in some other species the mode is not apparent. Plate LXXIII. flg. 6, 
and Part II. Plate XXIX. fig. 12. 
Halicnemia, Bowerbank. 
Skeleton formed of a single superior stratum of spicula radiating from the centre to 
the circumference of the sponge at about its middle, and of an inferior stratum of 
spicula distributed without order. 
The nearest alliance to this genus appears to be Tethea, in which the skeleton is 
formed of numerous fasciculi of spicula radiating from the centre to all parts of a sphe- 
rical or elliptical mass ; while in Ilalicnemia the radiating fasciculi are confined to a 
common plane, beneath which there is a second stratum of spicula, which fills the 
space beneath the radial stratum and the lower surface of the sponge, but without being 
disposed in order, and the spicula of the inferior stratum differ materially in form and 
proportions from those of the superior one. 
