LESSONS IN ZOOLOGY. 259 



The divisions mentioned are sometimes grouped together as 

 the invertebrata. 



(9) VERTEBRATA. The vertebrata are bilateral animals with 

 an internal cartilaginous or bony skeleton, prominent in which is 

 the vertebral column, which serves as a protection for the highly 

 developed nervous system. 



This scheme of classification is more or less incomplete and ar- 

 tificial. The knowledge of the animal kingdom is not sufficient 

 to enable zoologists to work out a natural system, if, indeed, such 

 a system exists in fact. 



The Protozoa, the first or lowest animals, like the protophyta, 

 live in the water, and so much alike are these lowest plants and 

 animals that in many cases it is difficult to determine whether a 

 given organism is an animal or plant. One of the simplest forms 

 of animal life is the amoeba. This minute animal may be found 

 on the submerged leaves of plants or in the mud at the bottom of 

 stagnant water. It appears much like a piece of jelly, nearly col- 

 orless, often granular, and more fluid in its central portion. 

 When active, they are constantly changing form , thrusting out 

 rays or arms, called pseudopodia, from one part or another of 

 the mass. In the larger pseudopodia the granular matter passes 

 in a stream toward the extremity and back again, the circulation 

 probably caused by the contractility of the protoplasmic mass. 

 By means of the pseudopodia the animal creeps slowly along, but 

 in no definite direction. The amoeba has no mouth or stomach; 

 it flows around its food, digests and absorbs such parts as it can 

 use, then flows away from the undigested parts. Besides the 

 granular matter they often contain a rounded body called a nu- 

 cleus, and sometimes a contractile vesicle. The amoeba multiplies 

 by simple division as in the case of protococcus. The amoeba, a 

 little mass of protoplasm without organs of any kind lives, 

 moves, absorbs oxygen, expels carbon dioxide, eats, grows and 

 reproduces its kind in short, appears to be a complete organism. 



The foraminiferae are much like the amoeba, except that they 

 secrete a calcareous shell. The earliest and oldest fossil known 

 IB supposed to be a member of this group. The shells of these 



