178 SYSTEMATIC ZOOLOGY FOR TEACHERS. 



divergence of form in different groups of animals, we will leave the subject to 

 follow it in those special groups. 



SECTION INVERTEBRATA. 



The animals grouped under this head are so very diverse in foim and 

 structure that they cannot, as a body, be defined by any positive characters 

 which are presented by all, hence negative characters must be used describ- 

 ing them. 



Invertebrates are animals which are not provided with a notochord, which 

 is a prolongation of nervous matter extending backward or downward from 

 an enlarged nervous body, called the brain, which is situated in the head, 

 and this prolongation is usually protected by a continuous chain of bones 

 known as vertebrae. 



PROVINCE III. 



Sponges, Poriferae. 



Sponges, which we now so cleary know as animals, were not mary 

 years ago regarded by most of our prominent naturalists, as belonging to the 

 vegetable kingdom. They were thought to propagate by freely swimming 

 spoors, much as seen in some of the sea weeds. Even after the discovery of 

 the cilia and collar cells and their accompanying whip-like pseudopodia, which 

 forever settled the question of the animality of the sponges, naturalists, net 

 fully comprehending the importance of these organs, placed the sponges 

 among the Protozoa. 



It was only as late as 1875 that Prof. A. Hyatt in his ''Revision of 

 the North American Poriferiae," raised the sponges to the rank of a province, 

 " equivalent structually to the Vertebrates or any of the large divisions which 

 a re characterized by the most important structural differences." 



No naturalist with any scientific training to-day doubts that sponges 

 ^o should be separated from the one-celled animals, yet in some ways they are 

 ^closely related to them. Some of the higher infusoria with long, whip-like 

 flagella are not unlike some of the sponge animals and all of the sponges in 

 the free swimming stage, closely resemble the ciliated infusoria. While any one 

 would be excusable for mistaking a young sponge in this freely moving 

 stage for an infusoria, no one, with our present advanced state of knowl- 

 edge, would mistake a sponge in an older, or fixed, condition of growth for 



