3504 THE SPONGES 



The walls of the canals are lined with flat cells, but in the flagellated chambers 

 the lining cells are more or less cylindrical, and each is provided at its free end with 

 a whip-like appendage or flagellum; and, further, the upper margin is expanded 

 into a thin hyaline collar, so that the whip appears to rise from the centre of a 

 basin or funnel. The currents of water traversing the body of the sponge are kept 

 up by the movements of the flagella of the collar cells. The flagella beat the water 

 in the flagellated chambers into the rootlets of the canals leading to the oscules. To 

 replace this, water flows into the flagellated chambers from the rootlets of the canals 

 passing down from the groups of pores in the skin. The currents entering the 

 sponge bring in oxygenated sea water, and minute food particles such as diatoms, 

 infusoria, etc. ; the currents from the oscules contain an excess of carbonic acid, of 

 waste products resulting from vital activity, and indigestible remains. The cells 

 lining the canals effect the exchange of gases, and take up food particles. 



At present, too little is known as to the physiology of digestion in sponges, to 

 permit of any definite statements being made. In some sponges, which have been 

 fed with carmine granules and then killed, the collar cells have been found loaded 

 with granules; in others, again, the flat cells lining the subdermal cavities 

 have been found gorged with the carmine. A terminal cluster of flagellated cham- 

 bers in the bread-crumb sponge may be compared to a hollow mulberry, reduced to 

 its skin but retaining its shape; each swelling represents one flagellated chamber, 

 opening by one wide orifice into the common central space which is continued into 

 the stalk. A grape-like cluster of mulberries would convey some idea of the ar- 

 rangement of the canals, in which the hollow main stem represents the terminal 

 oscular canal. Further, each swelling on the surface of the mulberry is perforated 

 by several round pores, termed prosopyles (entrances). Another call on the reader's 

 imagination must now be made. The openings of the infoldings on the surface are 

 closed over by a membrane perforated by pores. Suppose the mulberry cluster to 

 be immersed up to its stalk into a skin bag of jelly, and the skin to be tucked in 

 and folded so as to form channels or canals branching and diminishing in size, till 

 they abut on to the surface of the mulberries. Again, suppose the bag to be im- 

 mersed in water which, by some means, is made to enter the walls of the bag and 

 come out at the stalk. The current will pass from the pores in the skin, then along 

 the channels till it reaches the pores or prosopyles on the surface of the mulberries, 

 through which it passes, and proceeds to the stalks and main stem. The system of 

 canals from the skin pores to the prosopyles is termed in-current, and that from the 

 cavities of the mulberries to the orifice at the top of the stem out-current. This 

 structure is shown in the illustration, but the representation is extremely diagram- 

 matic. In a thin section of Halichondria one sees a labyrinth of in-and-out-current 

 canals and spaces together with small flagellated chambers; of the latter, often only 

 one or two open into an out-current space or rootlet. 



The jelly mass, which in the model supports the hollow mulberry cluster and 

 the channels from the outer skin, would of itself form an inefficient support, so we 

 must add to it a scaffolding of rods and bars or tough horny fibres; the common 

 bath sponge skeleton, and the skeleton of the Venus' s flower basket are the horny 

 and flinty scaffoldings supporting the soft tissues together with the flagellated 



