166 ANATOMY AND PHYSIOLOGY 



species, L. algoaensis, Bowerbank, MS., which is not un- 

 coinmon on specimens of Zoophytes and Fuci from Algoa 

 Bay and its neighbourhood. Figs. 349, 350, PL 

 XXVI. 



Oedeb II.— SILICEA. 



The genus Halichondria, as established by Fleming and 

 adopted by Dr. Johnston, when applied to the arrange- 

 ment of exotic as well as British species, embraces so wide 

 a range as to afford but little assistance in the determination 

 of species. Under this designation every known sponge 

 would be arranged having silex as the earthy basis of its 

 skeleton, however varied their anatomical structure might 

 be, excepting the few species contained in the genera 

 Geodia, Tethea, and 8'gongiUa. 



Dr. Johnston, in his ' History of British Sponges/ 

 has divided the British species into three sections, de- 

 pendent on their form, a character so mutable among the 

 Spongiadse, as to render it of little value under any cir- 

 cumstances, when unaccompanied by structural peculiari- 

 ties. I have therefore thought it advisable to distribute the 

 genera included in the order Silicea among seven suborders, 

 founded on the most striking peculiarities of the structure of 

 the skeleton. 



The first of these will consist of sponges having spiculo- 

 radiate skeletons. Skeletons not reticulated, but composed 

 of spicula radiating in fasciculi or separately from the base 

 or axis of the sponge. This order will contain as many as 

 fourteen distinct genera, the whole of which have skeletons 

 the spicula of which are arranged in radial order. The 

 mode of the radiation in these fourteen genera is not pre- 

 cisely the same, but they form three closely according 

 groups, of which the leading genus of each of the first two 

 may be considered as the type. 



