SPONGES. 691 



In the Sponge we have the very simplest type of Metazoan or multicellular 

 animal. The component cells of the body, which, in the higher Metazoa, 

 lose much of their individuality, functioning in masses (as tissues or organs) 

 for the good of the whole complex organism rather than as individuals for 

 their own benefit, are comparatively little modified in the body of the 

 Sponge, still acting individually almost more than collectively. 



The body wall of a Sponge consists of two layers. The outer layer is of a 

 gelatinous nature, and contains (1) cella which, like some of the unicellular 

 animals such as the Amoeba (Fig. 1, p. 714 ) , can change their 

 shape, throwing out processes in various directions ; and Constitution of the 

 (2) needle-like bodies, called spicules, which are formed Sponge Body, 

 by cells and give firmness to the wall and support the 

 apertures on its surface. The inner layer primarily consists of cells closely 

 resembling another form of unicellular organisms, the collared Flagellata 

 (Fig. 6, C, p. 718). Each of the collar cells in this layer is provided with the 

 whip-like appendage which characterises the Flagellata, so that the whole inner 

 surface of the wall of the simplest Sponge is lined with fine, waving whips. 

 The presence of these "whip cells" at one time led naturalists to consider 

 the Sponges as mere aggregates of unicellular animals. 



The very simplest type of Sponge, such as the one depicted in Fig. 1, is a 

 simple sac, generally more or less cylindrical, and attached by its narrowed 

 base to a rook or some other surface. This sac, which opens above by a 

 wide, circular orifice, the oscule, consists of an outer and an inner layer such 

 as have just been described. 



The water necessary to the life of the Sponge does not enter in at the 

 oscule, which must nut be considered as a mouth, but through multitudes of 

 much smaller apertures or pores that perforate the wall. 

 Within the cavity of the sac the collar cella of the lining layer 

 draw in from the water, which enters at the pores and streams 

 out at the oscule, small particles of nourishment, and for this 

 reason this space has been called the " gastral cavity." All 

 indigestible matter, together with the water containing it, is 

 ejected through the oscule. The whips with which the collar 

 cells are provided keep the water in constant motion, and help 

 to draw it in through the pores and to eject it through the 

 oscule. 



Other Sponges are less simple in structure, complexity 

 being brought about by the thickening of the walls of the eao 

 and by new arrangements of the whip cells. 



In a rather more complicated type of Sponge, the wall is 

 bulged out in the form of numbers of small radiating tubes. 

 The wall of each of these tubes is perforated by pores through ''f^^^^-^^'l^^ 

 which the water is drawn by the action of the whip cells, and ^p gpo^ag 

 is passed on into the central cavity with which all the tubes (Ascet a). 

 communicate. It is finally ejected through the oscule, which 

 has here the same position as in the simpler type of Sponge. In this second 

 type of Sponge the collar cella line the tubes only, the central cavity being 

 lined with flat cells. , 



In still more complicated types of Sponges with very thick walls an 

 elaborate canal system arises, leading the water through the thick walls to 

 the central cavity. The collar cells here no longer line either the centra 

 cavity or the canals, but form the lining of a great number of round 



