INVERTEBRATE ZOOLOGY. 33 



The ventral side is almost nnknown. Probably each thoracic segment has 

 a pair of crustacean-like legs, above which there maj- 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 Arachnoids, as 

 well as to the geniiine Crustacea. 



Class II. Arachnoidea Head and thorax fused into a single 



piece, the cephalo-thorax. bearing six 

 pairs of appendages, of which one is 

 prae-oral. These may all be used as legs, 

 or one or more pieces may be chelate or 

 toothed and serve as mandibles or wea- 

 pons of defense. In the lower forms the 

 abdomen is elongated and segmented and 

 may bear appendages, but in the higher 

 forms it is consolidated and may be fused 

 with the cephalo-thorax. Respiratory 

 organs originally lamellate gills, devel- 

 oped as adjuncts of the abdominal ap- 

 liendages. In the air breathing forms 

 they may be modified and reduced in 

 number, or even replaced by a sort of 

 tracheal system, not homologous with 

 that of insects. 



Sub-class I. Arachnoidea Branchiata. . . .Mainly fossil forms, all marine, 



gills lamellate, one pair of eyes ("trilo- 

 bite ") in side of cephalo-thorax, and one 

 pair of small ones anterior to these near 

 the middle line. Coxal joints of the legs, 

 or of some of them forming spiny plates 

 used in mastication. 



Order 1. Gigantostraka Fossil forms with long extended abdo- 

 men, which may terminate in a spine. 

 if Pterygotus. 



