218 ANATOMY AND PHYSIOLOGY 



sponges there are peculiarities of a similar description, such 

 as the presence of a reticulo-fibrous sheath, as represented 

 by Figs. 279, 280, Plate XVI, or the possession of 

 spines or tubercles of various forms, as represented in 

 ■JFigs. 275, 276, Plate XV, or of extraordinary modifica- 

 tions for prehension, as in the cidarate siliceo-fibrous 

 skeleton, represented also in the same Plate, fig. 278. 

 These and other similar structural peculiarities afford a 

 series of characters which are usually of a permanent and 

 very striking description. 



6. The Interstitial Membranes. 



The peculiarities of the interstitial membranes consist 

 principally in the shape and proportions of their tension 

 spicula, or of the forms and varieties of structure, and 

 mode of disposition of the retentive spicula. The latter 

 class of organs especially present a very extensive series of 

 striking characters that are essentially specific. In the 

 genera Halichondria, Isodictya, Hymeniacidon and others 

 containing numerous species, often very closely resembling 

 each other in all the principal structural characters, they 

 frequently, from the strongly marked peculiarities in their 

 form and proportions, present most valuable and decisive 

 specific characters, as in Figs. 255, 256, 257, 258, and 

 259, Plate XII. 



In AlcyonceUum and other genera the interstitial mem- 

 branes are strengthened and supported by layers of pri- 

 mitive fibrous tissue, arranged in parallel lines, and in 

 Stematumenia the same fibres abound, but they are not 

 disposed in the same symmetrical manner; and in some 

 sponges cellular structures are present in considerable 

 quantities. These tissues are all more or less valuable as 

 aids in specific distinction. 



7. The Intermarginal Cavities. 



The intermarginal cavities in the greater portion of the 

 Spongiadse are so indefinite in their form as to render but 



