24 FIRST LESSONS IN ZOOLOGY. 



softest; those of the Red Sea are next in quality, while 

 our West Indian species are coarser and less durable. Our 

 "West Indian glove-sponge corresponds to the Turkish 

 cup-sponge and Levant toilet-sponge of the Mediterranean. 

 The wool-sponge of Florida and the Bahamas is used as a 

 horse- or bath-sponge. 



The sponges are so unlike other many-celled animals 

 that they form a branch by themselves called Porifera. 



We see, then, that the sponge is composed of numerous 

 cells, which are arranged in layers and form tissues, but 

 it has no single definite mouth or stomach, ami the shape 

 of the body is indefinite. 



Cells grow by absorbing or taking in cell-food i.e., by 

 the assimilation of nutritious matter from without, and 

 this food may be in masses of considerable size when seen 

 under the microscope. Cells multiply by self-division. The 

 egg-cell of the sponge, and indeed of all. the higher ani- 

 mals, undergoes division of the yolk into two, four, eight, 



and afterward many cells; the cells 

 thus formed become arranged into 

 two layers or sets called germ-layers. 

 The outer is called the ectoderm, and 

 the inner the endoderm. A third 

 germ-layer arises between them, 

 called the niexndvnn, or middle germ- 

 , layer. From these germ-layers, or 



FIG. 20. - A two - layered "v 



germ, ec, ectoderm: *n, en- cell-layers, the tissues oi the body are 



doderm; , mouth opening 



into the digestive cavity, formed, such as muscle, bone, nerve, 

 and glandular tissue. These tissues form organs, hence 

 animals (as well as plants) are called organisms, because 

 they have certain parts formed of a particular kind of tissue 

 set apart for the performance of a special sort of work or 

 physiological labor. This separation of parts for particular 

 or special functions is called differentiation ; and the high- 

 est animals are those whose bodies are most differentiated, 

 while the lowest are those whose bodies are least differenti- 

 ated; hence high animals are specialized, and, on the other 



