STRUCTURE OF SPONGES. 



12' 



lines it the endoderm, or the outer covering layer the ectoderm. 

 In 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 hermaphrodite 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 sea- 

 weed, with a large exhalant aperture 

 at the opposite pole, and with 

 numerous minute inhalant pores 

 penetrating the walls. These walls 

 consist of (i) a flat covering layer; 

 (2) a mesogloea containing triradiate 

 calcareous spicules, phagocytes, and 

 reproductive elements; and (3) a 

 layer lining the central cavity, and 

 composed of collared flagellate cells, 

 like some of the monad Infusorians. 

 This simple sponge is not much 

 above the gastrula level ; it agrees 

 in its general architecture with a 

 simple Ccelenterate, such as Hydra, 



but differs from h in the absence Showing inhalant canalS) 

 of tentacles and stinging cells, and flagellate chambers, a 

 in the greater development of the 

 mesoglcea. 



More complicated forms. But a description of a simple 

 sponge like Ascetta conveys little idea of the structure of a 

 complex form such as the bath-sponge (Euspongia). 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 easily 

 and well. Like plants, they form buds, the outcome of 

 surplus nourishment. These buds, like the suckers of a 

 rose-bush, often acquire some apparent independence, and 

 the sponge looks like many vases, not like one. Moreover, 



FIG. 56. Section of 



sponge. After 

 Schulze. 



gastrula forming 

 mesoglcea, etc. 



F. E. 



in 



the 



