PHYLUM ARTHROPOD A 



209 



zoea in passing to the adult condition transforms through a more 

 advanced type of larva (Fig. 103) which is termed the megalops. 

 Six subclasses of the Crustacea are here considered: the 

 Trilobita, which are represented by fossil remains only, the 

 Phyllopoda, the Copepoda, the Ostracoda, the Cirripedia which 

 are exclusively marine, and the Malacostraca. 



Subclass Trilobita 



The trilobites are of importance because they represent a 

 primitive type of crustacean which has not persisted to recent 

 times but occurred in abundance during Cambrian times when 



Fig. 96. — A trilobite, Triarthrus Becki. A, dorsal; B, ventral aspect. {After 

 Beecher, from Zittell). 



the oldest fossil-bearing rocks were formed, reached an extreme 

 development in the Silurian, and disappeared in the Permian. 

 In habits, they were exclusively marine. Their remains are 

 among the most popularly known fossils. The body contains 

 a variable number of somites (Fig. 96) which are grouped into 

 head, thorax, and abdomen. Each segment bears a pair of 

 jointed appendages. The head bears five pairs all of which are 

 biramous except the antennules and they are simple. A pair of 

 compound eyes usually occurs on the head. 



A pair of longitudinal grooves separates the body into a central 

 axis and two lateral pleural lobes. This "trilobed" condition of 

 the body suggests the origin of the group name. 



About 2,000 species have been described. Proetus, Asaphus, 

 Dalmanites, Triarthrus (Fig. 96), Harpes, Ctenocephalus, Para- 



