100 ANATOMY AND PHYSIOLOGY 



In the canals deeply buried in the mass of the sponge 

 the sides frequently consist of but one layer of membrane 

 and primitive fibrous tissue, and in this case also the fibres 

 are always disposed at right angles to the axis of the canal, 

 but they are neither so numerous nor so closely packed as 

 in the sides of the great excurrent canals. 



The interstitial membranes are also furnished with these 

 fibres, sometimes in considerable quantity, but rather irre- 

 gularly disposed, while in other cases a single fibre only 

 will be observed meandering across the tissue. 



The interstitial membranous tissues in a beautiful little 

 specimen of Alcyoncellum from the North Sea, for which I 

 am indebted to my friend Captain Thomas, of the Hydro- 

 graphical Survey, are very similarly constituted to those of 

 the sponges of commerce. The membranous walls of the 

 interstitial cavities are each formed of a series of fibro- 

 membranous layers, the fibres of each layer being disposed 

 at angles varying from those above and below it. 



Pigs. 255, 266, 257, and 258, Plate XII, represent 

 portions of the lining membranes of the incurrent and ex- 

 current canals, and the mode of the disposition of the 

 primitive fibrous structure upon them. 



INTERMARGINAL CAVITIES. 



In the Halichondroid sponges, immediately beneath the 

 dermal membrane, there are numerous and, comparatively 

 speaking, large irregularly formed cavities, which receive 

 the water inhaled by the pores, and convey it to the 

 mouths of the incurrent canals, which have their origin in 

 the deepest portions of the spaces. These organs, from 

 their irregularity in size and form, are not always very 

 apparent, but if a section be made at right angles to the 

 surface in a dried specimen of Hdichondria panicea or 

 Chalina simulans, Bowerbank, they may be readily detected 

 and distinguished from the interstitial canals and" spaces of 

 the sponge. 



Pig. 300, Plate XIX, represents a section of Halichon- 



