134 



EVOLUTION AND GENETICS 



cated by fossils. The belemnoids persist in the one genus, 

 Spirilla. 



The Arthropoda. Another significant group of invertebrates is 

 the phyhnn Arthropoda (Fig. 73). In this phyhim a chitinous 

 exoskeleton is well developed, favoring fossilization. Another 

 distinctive characteristic, the presence of jointed appendages, 

 gives the phylum its name. The body is segmented. Because of 

 the high development of the group, it is looked upon as the culmi- 

 nation of evolution of seg- 

 mented invertebrates. 



The arthropods are at pres- 

 ent more successful than 

 any other group of animals 

 for three reasons; the same 

 may be said with equal truth 

 of one of the included classes, 

 the insects. Of these there are 

 three times as many species 

 as of all other animals to- 

 gether. Many of the species 

 produce immense numbers of 

 individuals, and finally, there 

 are almost no habitats on 

 earth of which the arthropods 

 have failed to avail them- 

 selves. 



Arthropod Ancestors. The 

 earliest organisms which show 

 arthropod characters of segmented body and jointed appendages, 

 with a hard exoskeleton, are the trilobites (Fig. 74). During the 

 Cambrian they were numerous, exceeding in abundance and diver- 

 sity all other kinds of animals. In the Devonian they began to de- 

 cline, and in the Permian became extinct. They were aquatic ani- 

 mals, as is shown by their association with marine deposits. They 

 are characterized by the division of the dorsal shield by two longi- 

 tudinal grooves into three parts, hence the name trilobite. The 

 body is divided transversely into an anterior head called the 

 cephalon, a series of segments forming the thorax, and a tail piece 

 or abdomen. The jointed appendages are biramous, as in the 

 Crustacea, with the exception of the anterior pair, the antennae. 



Fig. 73. — The crayfish, an arthropod. 

 (From Comstock's Introduclion to En- 

 tomology, with the permission of the 

 Comstock Pubhshing Company.) 



