OF THE SPONGIADiE. 97 



true, we have an intelligible solution to an infinite number 

 of phenomena among the lower classes of animals that 

 have hitherto been inscrutable. In the higher classes of 

 animal existence we know well that nervous energy is the 

 spring whence every other vital power proceeds, and we 

 trace the nervous system downward in the scale of animal 

 existences until from a few simple fibres it becomes obso- 

 lete ; and yet in those creations in which tubular nerves 

 are no longer to be detected life and action is as vivid in 

 proportion to their necessities as in the higher classes, 

 abounding in complicated nervous ramifications. Again, 

 then, we may ask, whence, in the absence of nerves, comes 

 the inspiration of all these vital actions, if it be not that 

 they are due to the inherent nervous properties of sarcode 

 — a never-failing material in animal existences. Every other 

 organ may in turn become obsolete, but sarcode never. It 

 continues its downward course in the chain of existence 

 until it at last becomes the sole representative of animal fife. 

 If under all these various conditions we consider its 

 modes of action, we shall find that its imbibing powers are 

 not exerted continually. In the Spongiadae, as in all other 

 animals, it has its intervals of action and of rest,* and this 

 habit will perhaps afford us a useful mode of distinguishing 

 between animals and vegetables. Thus in animals the 

 imbibition of nutriment is voluntary and at intervals, while 

 in vegetables it is involuntary and continuous. 



THE INTERSTITIAL CANALS AND CAVITIES. 



These organs exhibit their most complete mode of de- 

 velopment in the genus Spongia and in the Halichondroid 

 sponges, occupying nearly the whole of the masses of the 

 animals. They consist of two distinct systems, an incurrent 

 and an excurrent one. The incurrent series have their 

 origin in the intermarginal cavities immediately within the 

 dermal membrane, and their large open mouths receive 

 from these organs the water inhaled through the pores, 



* "Report on the Vitality of tlie Spongiadse," ' Brit. Assoc, lleports ' for 

 1S56, p. Ml, &c. - 



