228 AN INTRODUCTION TO ZOOLOGY 



bears three pairs of legs, and, with a few exceptions, one or 

 two pairs of wings. The abdomen has several or no append- 

 ages. The honeybee described in Chapter XII has been selected 

 as a type of this class. 



The Class Arachnida includes animals which have the head 

 and thorax fused into a cephalo-thorax. They possess four pairs 

 of legs, an abdomen usually without appendages, and a heart in 

 the dorsal part of the abdomen. Such diverse animals as the 

 spider, scorpion, mite, and King crab are placed together in this 

 class. The probable affinities of the various classes of arthro- 

 pods are shown in Figure 123. 



The Biogenetic Law. In concluding this chapter we wish 

 to refer to a law which has commanded the attention of 

 zoologists for almost a century; namely, the law of bio- 

 genesis, also known as the recapitulation theory. Organic evo- 

 lution, that is, the evolution of one organism from another, 

 is accepted as an established fact by practially all zoologists 

 at the present time. This fact of organic evolution is 

 expressed in the diagram of the principal phyla of the 

 animal kingdom on page 7, where the various groups appear 

 to be derived from a central stem which represents a 

 series of ancestral forms. From an examination of the phyla 

 represented in our diagram we gain the fact that the members of 

 each group are more complex than those of the group just beneath 

 them on the page. Evolutionists do not claim that the more 

 complex forms have evolved directly from the simpler animals, 

 but that their ancestors were related. Beginning with the sim- 

 plest animals we find that a single cell performs all the necessary 

 processes of life, e.g. Ameba. Within the lowest phylum, the 

 Protozoa, there are animals consisting of a number of cells more 

 or less intimately bound together into a hollow spherical colony, 

 e.g. Volwx. Passing to the next higher group of organisms we are 

 introduced to animals that possess two layers of cells, surrounding 

 a single cavity, e.g. Hydra. All animals above the Ccelenterates 

 have three layers of cells forming their body walls, i.e. are triple- 



