OF THE SPONGIADjE. 83 



condition, nor in a state of partial decomposition, have I 

 ever been able to satisfy myself of the existence of a sur- 

 rounding membrane. It appears to me that these bodies 

 are the result of a natural resolution of the sarcode into 

 granular masses of various sizes, each of which, on being 

 liberated from the parent body, becomes an independent 

 gemmule, which is capable of reproducing the species of 

 sponge from which it emanates. And I have long suspected 

 that the Amoebae found in ponds and rivers, and also in 

 sea-water, are not in reality distinct species of animals, but 

 that they are free portions of the sarcode of various species 

 of Spongiadae. 



ORGANIZATION AND PHYSIOLOGY. 



Previously to entering on the subject of the organization 

 and physiology of the Spongiadae in detail, it will be neces- 

 sary to take a brief view of the general structure of these 

 animals. Whatever may be their form, or however they 

 may differ from each other in appearance, there are certain 

 points in their organization in which they all agree. In 

 the first place, however variable in its form and mode of 

 structure, there is always a skeleton present on which the 

 rest of the organic parts are based and maintained. Amidst 

 the skeleton, and intimately incorporated with it, are the 

 interstitial canals, consisting usually of two series ; the first 

 appropriated to the incurrent streams of the surrounding 

 water, and the second to the excurrent streams, which they 

 conduct from the interior of the sponge to the oscula at its 

 surface, through which they are discharged. In the event 

 of the absence of the excurrent system of canals, their office 

 is served by the great cloacal cavities that are found to exist 

 in some forms of sponges, extending from the base to the 

 most distant parts of the animal. Beside these large cavi- 

 ties, there are others of a much more limited character, 

 the intermarginal cavities, which are situated immediately 

 below the dermal membrane, and which receive the water 



