DESCRIPTION OF A SIMPLE SPONGE. 



117 



species. It is traversed by canals, through which currents of 

 ivater bear food inwards and waste outwards. Numerous 

 minute pores on the surface open into afferent canals, leading 

 into a cavity or cavities lined by endoderm cells, many or all 

 of which are flagellate. To the activity of the flagella the all- 

 important water currents are due. The endodermic or gastric 

 cavity may be a simple tube, or it may have radially outgrow- 

 ing chambers, or it may be represented by branched spaces, 

 from which efferent canals lead to the exterior. Where there 

 is a distinct central cavity there is usually 

 but one large exhalent aperture (osculum), 

 but in other cases there are many exhalent 

 apertures. 



The ectoderm is the least important 

 layer; it covers the body, and is perhaps 

 continued into the afferent canals; the 

 endoderm lines most of the internal cavities, 

 and is typically flagellate ; the intervening 

 mesoglcea contains a skeleton of lime, flint, 

 or spongin ; amoeboid cells or phagocytes, im- 

 portant in digestion and excretion ; repro- 

 ductive cells, and other elements. 



Budding is very common, and in a few 

 cases buds are set adrift. Both herma- 

 phrodite and unisexual forms occur. The 

 sexually produced embryo is almost always 

 developed within the mesoglcea, and leaves 

 the sponge as a ciliated larva. With the 

 exception of one family, all are marine. 



Description of a Simple Sponge. 



A very simple sponge, such as Ascetta, 

 is a hollow vase, moored at one end to rock or seaweed, 

 with a large exhalent aperture at the opposite pole, and 

 with numerous minute inhalent pores through the walls. 

 These walls consist of (i) a flat ectoderm ; (2) a mesoglcea 

 containing triradiate calcareous spicules, phagocytes, and 

 reproductive elements ; and (3) an endoderm lining the 

 central cavity, and composed of collared flagellate cells, 

 almost exactly like some of the monad Infusorians. This 

 simple sponge is not much above the gastrula level ; it 



FIG. 31. Simple 

 Sponge (Ascetta 

 primordialis}. 



(After H^CKEL.) 



Note the vase-like 

 form, the apical oscu- 

 lum, the inhalent pores 

 in the walls. 



