AND PHYSIOLOGY OF THE SPONGTADTR. 
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gi-eat number of strongly marked specific distinctions, arising not only from their varie- 
ties of form and proportion, but also from their relative positions in the dermal crusts 
of those genera where they most abound ; and their modes of disposition and connexion 
with each other are also very characteristic. 
8. The Interstitial Canals and Cavities. 
These organs themselves present very few characters that are of much service in spe- 
cific descriptions, but their subsidiary spicula are often very suggestive of the nature 
and character of the species. Of this description, are the recurvo-ternate spicula in the 
interstitial cavities immediately beneath the dermal crust of some species of Geodia, 
and just without the dermal membrane of Tethea cranium', the remarkable groups of 
recuno-quatemate spicula, represented by fig. 10, Plate XXX., Part II. ; the trenchant 
bihamate spicula of Hymedesmia Johnsoni, figs. 1 & 2, Plate XXXI. ; and many other 
instances of offensive or defensive spicula, either disposed in groups or singly, in these 
canals or cavities. 
9. The Cloacal Cavities. 
The cloacal cavities are especially valuable and characteristic in the calcareous 
sponges. Their position, number, extent, and form — the number and position of their 
excurrent orifices — the mode in which those orifices are armed and the nature of that 
armature, or the entire absence of such defences — the internal defensive spicula, their 
varieties of form, and mode of arrangement, — all these characters are highly effective 
and valuable as specific descriptions. In other genera of sponges the cloacae afford 
striking and very effective distinctions, especially in Alcyoncellum, Polymastia, Halyphy- 
sema, and Hyalonema. Among the Keratosa also they avail to a considerable extent ; 
but the latter order does not afford us the same wide range of striking characters that 
exist so abundantly in the cloacae of the order Calcarea. 
10. The Barcode. 
The universal presence and similarity in structure of the sarcode of the Spongiadae 
renders the range of its use as a specific character very limited; but the spicula im- 
bedded in its substance so abundantly in many species are so various in form, and so 
strikingly distinct from each other, as to afford a most valuable series of discriminative 
characters. 
The greater portion of these spicula are more or less stellate in form. They vary in 
shape to a considerable extent in each group, in consequence of incomplete or com- 
plete development, and the number of the radii in the stellate forms is in many cases 
very uncertain; but although this amount of variation exists in each of the separate 
forms, there is always a limit to these differences, and a normal character present which 
renders it by no means difficult to decide to which class they belong. Independently of 
the pecuhar characters of their own form and modes of radiation, their radii are fre- 
MDCCCLXII. 7 0 
