OF THE SPONGIAD^. 319 



little service in the distinction of species ; but in Geodia, 

 Pachymatisma, and a few other genera their structure is 

 very much more regular, and their form, proportions, and 

 mode of disposition afford good characters. But although 

 of no extensive essential value themselves, their subsidiary 

 ternate spicula present a great number of strongly marked 

 specific distinctions, arising not only from their varieties 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 connection 

 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 specific descriptions, but their 

 subsidiary spicula are often very suggestive of the nature 

 and cjiaracter of the species. Of this description are the 

 recurvo-ternate spicula in the interstitial cavities imme- 

 diately beneath the dermal crust of some species of Geodia, 

 and just without the dermal membrane of Tbthea cranium ; 

 the remarkable groups of recurvo-quaternate spicula, repre- 

 sented by Fig. 392, Plate XVIII ; the trenchant bihamate 

 spicula of Hymedesmia Jolinsoni, Eg. 112, Plate V, and 

 Pig. 293, Plate XVIII; 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 charac- 

 teristic 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 



