66 ANATOMY AND PHYSIOLOGY 



to proving the animal nature of the Spongiadae, the 

 evidence afforded by the coincidence of its structural 

 character and its chemical constituents with those of Sertu- 

 laria operculata, are still more conclusive than that derived 

 from the chemical constituents of silk ; and, in truth, 

 the action of the chemical agents on the zoophyte and the 

 sponge, as might naturally be expected, are almost in 

 perfect accordance. 



MEMBRANOUS TISSUES. 



These structures may be divided into two classes : 



1st. Simple membranous tissue. 



2nd. Compound membranous tissue. 



The first is a simple, apparently unorganized, thin, 

 pellucid tissue. It is evidently not composed of an exten- 

 sion of keratode, as it is rapidly decomposed after the 

 death of the animal. It is found in abundance filling up 

 the areas of the network of the skeleton in a great variety 

 of sponges, and it appears to be capable of secreting sarcode 

 on both its surfaces when thus situated ; on the dermal 

 membranes the sarcode is found on the internal sur- 

 face only. 



Compound membranous iissties. These structures con- 

 sist of simple membranous tissue combined more or less 

 with primitive fibrous tissue. Their most simple forms 

 exist in the membranes lining the interstitial cavities of the 

 sponge, and in the dermal membranes. 



It is difficult in some cases to discriminate between this 

 class of tissues and simple membranes, unless it be by the 

 aid of their functional characters, as the compound tissues 

 are frequently quite as pellucid, although not so thin, as 

 the simple ones. 



In dermal membrane, and the membranous linings of 

 the internal cavities of the sponge, they are thin and 

 very translucent ; but by a careful examination with high 

 microscopic powers and transmitted light, with the aid of 

 polarization, we frequently detect the elastic primary 



