26 A SYNOPSIS OF ANIMAL CLASSIFICATION. 



SUPPLEMENTARY CLASS, TRILOBITEA. 



An important group of fossil forms, presenting some superficial 

 resemblance to the Isopods. They are oval and flattened in form, and 

 possess a dorsal carapace, divided by two longitudinal grooves into three 

 areas, one median {rhachis) and two lateral {pleura). The carapace 

 consists of a cephalo-thorax, behind which are a varying number of 

 freely movable somites sometimes called the " thorax" and terminated 

 by a plate-like " pygidium," formed of several fused somites. 



The ventral side is almost unknown. Probably each thoracic somite 

 had a pair of crustacean-like legs, above which there may have been a 

 pair of gills, protected by the overhanging edge of the carapace. Recently 

 (1894) some finely preserved specimens have been discovered showing 

 one pair of long antennae. 



The Trilobites appear to have close affinity to the branchiate Arach- 

 noids, as well as to the genuine Crustacea. 



Class II. TRACHEATA Terrestrial Arthropods, with one 



pair of antennas; appendages 

 numerous in the lower members 

 of the class; in the higher ones 

 few and definite in number. Res- 

 piration by means of branching 

 tracheal tubes opening to the ex- 

 terior by means of spiracles, placed 

 along the sides of the body. A few 

 are secondarily adapted for aquatic 

 life, and breathe by numerous de- 

 vices, mostly modifications of the 

 tracheal system. 



Sub-class I. PERIPATOIDEA A group consisting of a very few 



tropical forms, isolated from one 

 another, and found in all conti- 

 nents. In many respects they are 

 more like Annelids than Arthro- 

 pods, and form an interesting con- 

 necting link between the former 

 and the genuine Tracheata. The 

 body somites are all alike and the 

 appendages, which are creased or 

 folded rather than jointed, show 

 no regional differentiation. Each 

 somite has a pair of nephridia. 



