DESCRIPTION OF A SIMPLE SPONGE. 



117 



species. It is traversed by canals, through ivhich cm-rents of 

 water hear food inwards and waste outwards. Numerous 

 minute pores on the surface open into afferent canals, leaditig 

 into a cavity or cavities lined by endoderm cells, man v or all 

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

 i?7iportanf water currents are due. The endodermic or gastric 

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

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

 from ivhich efferent canals lead to the exterior. Where there 

 is a distifict 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 

 mesoglaa contaitis a skeleton of lime, flint, 

 or spongin ; ammboid cells or phagocytes, im- 

 portant in digestion and excretion ; repro- 

 ductive cells, and other elements. 



Buddiitg is very common, and in a few 

 cases buds are set adrift. Both herma- 

 phrodite and unisexual for7ns occur. The 

 sexually produced embryo is almost always 

 developed within the mesoglcea, and leaves 

 the sponge as a ciliated larva. IJ'ith the 

 exception of one family, all are marine. 



Fig. 31. — Simple 

 Sponge {Ascetta 

 primoi'dialis). 



(After H/ECKEL. ) 



Note the vase-like 

 form, the apical oscu- 

 lum, the inhaletit pores 

 in the walls. 



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 mesogkea 

 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 



