HALICHONDRIA. 89 



ther by such insensible gradations, that the differences can scarce- 

 ly be deemed of generic value, though I have availed myself of 

 them in distributing the species into sections. 



The essential character of Halichondria is the existence of 

 siliceous spicula in the fibre or parenchyma of the sponge, not 

 collected into radiating fascicles but equally distributed through- 

 out its homologous body. " The spicula are generally united 

 into fasciculi by an enveloping glutinous or condensed cellular 

 substance, and by the junction of these fasciculi in various modes, 

 fibres are formed which traverse every part of the body, form- 

 ing the boundaries of canals and orifices, and giving form and 

 support to the whole of the gelatinous or soft cellular substance 

 of the animal." — (Grant, Comp. Anat. p. 5.) These spicula, 

 so far as the British species are concerned, seem to be always 

 in the shape of simple needle-like crystals, nor does any species 

 present us with spicula of two different forms, though they some- 

 times vary much in length and gracility, but I cannot assent to 

 the opinion of Dr Grant, that the form is different in every dis- 

 tinct species. Were this so, the task of defining them and track- 

 ing them through their protean guises would be easy, but my 

 present experience makes me suspicious of a test, the value of 

 which has not been proved by any extensive series of examina- 

 tions. 



The structure and consistency of the sponge is nearly uniform 

 throughout the mass, but in many species the texture is closer 

 and denser at the surface, which often looks, after being dried, as 

 if it were coated over with a white skin or membrane. The 

 glutinous fluid is usually sparing in quantity, so that dried speci- 

 mens exhibit in general faint traces of its existence. 



It is in Halicondria that the circulation peculiar to the spon- 

 ges has been principally observed, and the cmstaceous species are 

 best adapted for the study of its phenomena. A single observa- 

 tion is sufficient to convince us that this circulation has nothing 



