330 THE AMERICAN MONTHLY (Dec 



sea-weed, and in this case the weed and s[)onge can be 

 transferred together to the vessel, thus affording a better 

 opportunity for examination than if separated from its 

 support. 



On reaching home, we shall transfer the sponge to a 

 watch-glass or shallow glass vessel, so that it is well with- 

 in focus of the hand lens, and after the sponge has be- 

 come accustomed to its new surroundings a current of 

 water will be seen issuing from the top of it. This will 

 be the more evident if a pinch of powered carmine is ad- 

 ded to the water, and as the current is fairly constant 

 hour after hour, it is evident that its source must be con- 

 stant too. If the hand lens is powf ul enough, smaller cur- 

 rents will be seen setting in towards the sides of the 

 sponge and these form the source of supply to the larger 

 current. The actual course taken by the current is not 

 very evident, however, till we remove the sponge from 

 the water, and with a sharp razor make a longitudinal cut 

 through the very centre of it. This reveals the fact of a 

 central cavity, the gastral cavity, extending from the bot- 

 tom to the top of the sponge, the cavity being closed at 

 the bottom and open at the top. Another fact, not quite 

 so evident, however, can be made out, namely, that there 

 are tiny breaks in the continuity of the wall which lead 

 into minute open spaces. This rough-and-ready method 

 of examination will not furnish much information about 

 the actual direction of the current, but more exact work 

 with proper appliances reveals much more, and we are able 

 to trace, step by step, the exact course of a stream of wa- 

 ter through the tiny openings or pores in the skin of the 

 sponge to the central cavity — in fact, to establish a defi- 

 nite canal-system which may be regarded as the circula- 

 tory system, commencing at the pores and ending at the 

 outlet at the top of the sponge tlirough what is known as 

 the osculum. In man the circulatory system is a closed 

 one, in the sponge it is open ; in man oxygen and nutrient 



