CHAR A CTERISTICS. 5 2 9 



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 

 chambers 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 arrangement 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 (en- 

 trances). 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 immersed 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 incurrent, 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 diagrammatic. 

 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' flower-basket are the horny 

 and flinty scaffoldings supporting the soft tissues together with the flagellated 

 chambers and the channels which lead to and from them in the living sponges. 

 The fleshy or jelly substance of sponges is termed mesoderm (middle layer), 

 because it is situated between the collar-cells, which constitute the endoderm 



vol. vi. — 34 



