24 FIBST LE880N8 IN ZOOLOGY. 



softest; those of the Eed 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 Forifera. 



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

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

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

 of The hody 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 



/<W2dljQ%X thus formed become arranged into 

 ecj^^^OT^^^x^i two layers or sets called germ-layers, 

 raffl^ ^W*^^ ^^^ outer is called the ectoderm, and 

 *"^^fei , ^^Q ^-^^ inner the endoderm. A third 

 ^^^^ i (S(^5^ gsrm-layer arises between them, 

 \^/'"j'"Xl-^ called the mesoderm, or middle germ- 



Fig. 80. -a tv.o - layered %ei'- ^^0^^ ^^^sc germ-layers, or 

 germ, ec, ectoderm; -n, en- ccll-layers, the tissucs of the bodv are 



aoderm; a, mouthopenmg J ' j 



into the digestive cavity, formed, such as muscle, boue, 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 



