414 ARTHROPODA. 



with several pairs of schizopodal swimming feet. There are two 

 large compound eyes and, dorsal to the intestine, a heart. Fre- 

 quently the carapace is armed with very long spines projecting 

 from front, back, and sides, which are intended as protection from 

 enemies. 



Nauplius and zoea are of systematic importance, since they 

 rarely both appear in the life cycle of one individual. The nau- 

 plius is characteristic of the lower Crustacea the t Entomostraca.' 

 The zoea, on the other hand, has never been noticed in the En to- 

 rn ostraca, but occurs in many Malacostraca. A nauplius appears 

 in only a few Malacostraca, like the schizopods and Peneus, and 

 there precedes the zoea stage. It must not be forgotten that 

 many forms among both Entomostraca and Malacostraca have no 

 zoeal or nauplius stage. 



Frequently the lower Crustacea are united under the name 

 Entomostraca, but, aside from the nauplius stage and the posses- 

 sion of a shell gland, the only characters of the group are negative. 



Sub Class I. TriloUtce. 



The most important fossils of the class of Crustacea are the 

 Trilobites which appeared in the Cambrian and died out in the 

 Permian, being extremely abundant in the Silurian. The body 

 (fig. 416) consists of head and trunk, the latter segmented. In 

 the young the segments are very few, but increase in number with 

 age (10-29, according to the species). The hinder segments fre- 

 quently differ from the rest and form an abdomen OTpygidium. 

 Dorsally the animal is divided by two grooves into three lobes, 

 marking off in the head a glabella and two gence; in the trunk 

 rhachis and two pleurm. On the head there are usually a pair of 

 compound eyes, which in the young were frequently ventral, but 

 are brought to the dorsal surface with growth. For many years 

 little was known of the under surface, but lately specimens of 

 Triarthrus becki (fig. 417) from the Utica slate have revealed the 

 appendages. On the head are a pair of simple antennae, and four 

 pairs of schizopodal feet, the bases of which acted as jaws. It is 

 a question whether the first pair of jaw feet correspond to the sec- 

 ond antennas or whether these have been lost in the group. The 

 trunk segments bear biramous feet. 



In some respects the trilobites resemble the Xiphosura (infra), 

 but the possession of antennas and biramous feet place them among 

 the Crustacea. Here their position is very uncertain. We have 



