STRUCTURE OF SPONGES 



155 



and represent a cul de sac in evolution. There are no nerve- 

 cells — a fatal defect. 



Budding is very common ^ and in a few cases buds are 

 set adrift. Both herma- 

 phrodite and unisexual forms 

 occur. The sexually-pro- 

 duced embryo is almost 

 always developed within the 

 mesogloea, and leaves the 

 sponge as a ciliated larva. 

 Except the family of Spon- 

 gillids, 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 aper 



Fig. 74.— Sponge spicules. 



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

 raxon ; 5, anchor ; 6, polyaxon ; 7, a 

 kind of amphidisc. 



ture at the opposite pole, and with 

 numerous minute inhalant pores 

 penetrating the walls. In the calca- 

 reous sponges, the pores are minute 

 perforations in single cells (poro- 

 cytes). 



The walls consist of — (i) a flat 

 covering layer ; (2) a mesogloea con- 

 taining triradiate calcareous spicules, 

 phagocytes, and reproductive ele- 

 ments ; and (3) a layer hning the 

 central cavity, and composed of 

 collared flagellate cells, like some of 

 the monad Infusorians (cf. Fig. 71). 



More complicated forms. — But 

 a description of a simple sponge like 

 Ascetta conveys little idea of the 

 structure of a complex form such as 

 "■flSS.e'°'"chamber';!"""a the bath-sponge (FAcspongiu). Let us 

 gastruia forming in the cousidcr the Origin of compUcatious 



mesogloea, etc. / \ r~i i j j 



{a) Sponges — long regarded as 

 plants — are plant-like in being sedentary and passive. 

 They seem also to feed easily and well. Like plants, 



Fig. 75. — Section of 

 sponge. — After F. 

 Schulze. 



