ANNULOSA : CRUSTACEA. 219 



the extinct Trilobites. In the typical genera Limnadia and 

 Apus the body is protected by a carapace, which is bivalve 

 in the former and shield-like in the latter. In Limnadia the 

 carapace covers the greater part of the body, and opens along 

 the ventral margin. There are from 18 to 30 pairs of mem- 

 branaceous and respiratory feet. In Apus the carapace is 



Fig. 67.— Phyllopoda. Fairy Shrimp (Chifocephalus liia^Aanus)— after Baird. 



clypeiform and covers a portion of the abdomen; and there 

 are sixty pair of feet, of which all but the first pair are 

 foliaceous. Apus is gregarious, fresh-water in habit, and often 

 found in great numbers in. pdols and ditches in Europe. The 

 different species oi Branchipus have the body unprotected by 

 any carapace, and are found in ponds and swamps in various 

 parts of the world. The various " Brine-shrimps " (Artemia) 

 are found inhabiting the brine-pans in salt-works, or occur in 

 salt-lakes in both hemispheres, being especially abundant in 

 Great Salt Lake in Utah. 



Order III. Trilobita. — This order is entirely extinct, none 

 of its members having survived the close of the Palasozoic 

 period. It is probable that the Trilobites should be placed 

 near the Phyllopoda; but their exact position is uncertain, as," 

 with one exception, no traces of any appendages of any kind, 

 except the labrum, have hitherto been discovered in any Tri- 

 lobite. 



The body of a Trilobite (fig. 68) was covered with a "crust,"' 

 or exoskeleton, which shows more or less markedly a division 

 into three longitudinal lobes, from the presence of which the 

 name of the order is derived. The shell is composed of a 

 cephalic shield, a certain number of free and movable thoracic 

 rings, and a caudal shield, or " pygidium," the rings of which 

 are more or less completely anchylosed. On the under surface 

 of the body nothing has hitherto been discovered, except the 

 '' hypostome," or " labrum," which was a plate placed in front 

 of the mouth. No traces of ambulatory or natatory limbs, of 

 branchiae, or of antennas, have ever been discovered. The 

 eyes, when present, are compound, and usually sessile, but 

 11 



