PHYSIOLOGY OF THE INVERTEBRATA. 



3' 



all these animals possess a permanent alimentary cavity, lined 

 by a special layer of cells." 



The histological structure of an adult sponge is comparable 

 in certain details to the Amoihcv and to the Infusoria Jlagd- 

 lata, for adult sponges are partly composed of aggregations 

 of amcebiform cells and partly of flagellate cells. But in the 

 embryonic condition a sponge is comparable to an embryo 

 Jli/drozoon, and is consequently unlike any form belonging to 

 the Protozoa. 



The body of these animals has a spongy consistence, and is 

 usually strengthened by a calcareous, silicious, or fibrous 

 skeleton.* All over the surface of the body are minute 

 inlialent apertures, through vs^hich the water, bearing food 

 particles, passes into the gastro-vascular space or body-cavity. 

 This body-cavity is lined internally with flagellate cells. 



Besides the inhalent apertures, there may be one or many 

 exhalent apertures (oscula). The former are comparable to 

 the intercellular spaces of plants, and are formed by the 



Fig. 4. — Diagram of Section of Spongilla. 



{After Huxley.) 



A, rt = inhalent apertures, b = exhalent aperture ; the arrows indicate 

 the direction of the currents. B = an endoderm cell. 



separation of one cell from another. These apertures or 

 pores are not constant, for "they may be temporarily or per- 

 manently closed, and new ones formed in other positions." 

 The waste materials or excretory matters of each cell are 



* The MyxosponyiK are devoid of a skeleton. 



