168 



GENERAL ZOOLOGY 



Trilobites. Some of the most abundant of the forms in 

 the earlier periods of life on the earth were the tri'lobites 



(Pha'cops cauda' tus, Fig. 94), which are 

 an extinct type of the Crustacea. They 

 are named trilobite from the apparent 

 division of the body longitudinally into 

 three parts. The length of the body 

 varied from a fraction of one inch to two 

 feet. They are thought to have lived in 

 shallow water along shores, and through 

 a long period of the earth's history must 

 have been a most characteristic feature 

 of the fauna of the world. Of the many 

 species of trilobites not one remains liv- 

 ing at the present day. 



Definition of Crustacea (Lat. crusta, 

 "shell"). The crayfish and the species 

 mentioned thus far in this chapter belong 

 to the class Crusta'cea. They are constructed on the plan 

 of a series of body divisions (somites) seldom exceeding 

 twenty in number. The somites 

 are usually bilaterally symmetri- 

 cal (uniform in structure on either 

 side the median line). Typically 

 each somite has a pair of branched, 

 jointed appendages. Two pairs of 

 antennae are present. The exo- 

 skeleton contains chitin and car- 

 bonate of lime. Crustacea are 

 essentially aquatic in their habits 

 and, with the exception of the 

 lowest forms, breathe through 



gills. Nine tenths of the class are said to live in the ocean, 

 some in fresh water, and relatively few species on land. 



Fig. 94. Trilobite 



Taken from a report 

 of the Geological Sur- 

 vey, United Kingdom 



Fig. 95. Zoea stage of crab. 

 (Much enlarged) 



After Emerton 



