OF THE SPONGIAD^. 99 



the intervening space being occupied by the interstitial 

 cavities, the interior forming one large cloacal cavity, 

 which discharges its contents through a cribriform mouth 

 ait its distal end. In Grantia both systems, the incurrent 

 and excurrent interstitial canals, become very nearly obso- 

 lete, the large intermarginal cavities or cells imbibing the 

 water through their pores on the distal extremities, and 

 becoming enlarged and elongated until they reach the 

 parietes of the great central cloaca, into which they dis- 

 charge their contents, each through a single osculum, into 

 a short depression or cavity in the parietes of the great 

 cloaca, and this shallow cavity represents the nearly obsolete 

 system of excurrent canals. 



The membranes lining the incurrent and excurrent canals 

 are frequently highly organized. In the common honey- 

 comb sponge of commerce, when in the same condition as 

 when taken from the sea, these canals are constructed of a 

 series of compound membranes, each consisting of simple 

 interstitial membrane with a layer of primitive fibrous tissue 

 beneath it ; the fibrous portion consisting of a single series 

 parallel to each other, and so closely adjoining as to touch 

 each other through nearly their whole course (Fig. 255, 

 Plate XII). 



When the fibres are clear of the membranous tissue 

 they appear as simple pellucid threads, but when covered 

 by the membrane they frequently appear as if monihform ; 

 this character seems to be due to minute molecules arranged 

 in linear series on the membrane immediately above them. 

 These membranes abound in large, open, oval spaces, so that 

 the tissue assumes very much the appearance of areolar 

 tissue, as described by Professor Bowman in his treatise on 

 mucous membrane in the ' Cyclopaedia of Anatomy and 

 Physiology.' 



The layer of membrane forming the surface of the canal 

 has its fibres disposed at right angles to the axis of the 

 canal, while those of the layers beneath it assume various 

 directions, usually in straight lines, excepting in the vicinity 

 of the areas of communication, around which they curve to 

 strengthen their margins. 



