PHYSIOLOGY OF THE INVERTEBRATA. 31 



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 Amosbce and to the Infusoria flagel- 

 lata, for adult sponges are partly composed of aggregations 

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

 embryonic condition a sponge is comparable to an embryo 

 Hydrozoon, 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 

 inhalent apertures, through which 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 



I. 



Fig. 4.— Diagram of Section of Spongilla. 



(After Huxley.) 



A, fl = 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 Myxospongim are devoid of a skeleton. 



