126 



PHYLUM PORIFERA SPONGES. 



FIG. 58. Sponge spicules. 



Monaxon ; 2, triod ; 3, triaxon ; 4, 

 tetraxon ; 5, anchor: 6, polyaxon ; 

 7, a kind of amphidisc. 



fact) the sponges are very different from other Metazoa^ and 



represent a cul de sac in evolution. 



Budding is very common , 

 and in a few cases buds are 

 set adrift. Both hermaph- 

 rodite and unisexual forms 

 occur. The sexually-produced 

 embryo is almost always 

 developed within the mes- 

 ogloza, 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 exhalant aperture 



at the opposite pole, and with 



numerous minute inhalant pores 



penetrating the walls. In the 



calcareous sponges, the pores are 



minute perforations in single cells 



(porocytes). 



The walls consist of (i) a flat 



covering layer; (2) a mesogloea 



containing triradiate calcareous 



spicules, phagocytes, and reproduc- 



tive elements ; and (3) a layer lining 



the central cavity, and composed of 



collared flagellate cells, like some 



of the monad Infusorians (cf. Fig. 



55)- 



More complicated forms. But a 

 description of a simple sponge like 

 Ascetta conveys little idea of the 



FIG. 59. Section of a 

 sponge. After F. E. 

 Schulze. 



r i r i Showing inhalant canals, 



Structure Of a COmpleX form SUCh aS flagelfate chambers, 



the 



the bath-sponge (Euspongid). Let us 

 consider the origin of complications. 



(a) Sponges long regarded as plants are plant-like in 

 being sedentary and passive. They seem also to feed 



