ARTIIROPODA. 287 



Cray-fish (Astacus), Lobster (Homarus), and Crab (Can- 

 cer). Crabs differ from Lobsters chiefly in being formed 

 for creeping at the bottom of the sea instead of swim- 

 ming, and in the reduction of the abdomen or " tail" to a 

 rudiment, which folds into a groove under the enormous 

 thorax. They are the highest and largest of living Crus- 

 tacea: they have been found at Japan measuring fifteen 

 feet between the tips of the claws. 



CLASS II. Arachnida. 



The Arachnids are closely related to the Crustaceans, 

 having the body divided into a cephalo-thorax and abdo- 

 men. 148 To the former are attached eight legs of seven 

 joints each ; the latter has no locomotive appendages. 

 The head carries two, six, or eight eyes, smooth and ses- 

 sile (i. e., not faceted and stalked, as in the Lobster), and 

 approaching the eye of the Vertebrates in the complete- 

 ness and perfection of their apparatus. The antennae, if 

 present, are only two, and these are not "feelers," but 

 modified to serve for the prehension of food. 149 They are 

 all air-breathers, having spiracles which open either into 

 air-sacs or tracheae. The young of the higher forms un- 

 dergo no metamorphosis after leaving the egg. 



Arachnids number nearly five thousand species. The 

 typical forms are divided into three groups : 



1. Acarina, represented by the Mites and Ticks. They 

 have an oval or rounded body, without any marked artic- 

 ulations, the head, thorax, and 

 abdomen being apparently 

 merged into one. They have 

 no brain; only a single gan- FlG m _ A Mite 



glion lodged in the abdomen. rum), one of the lowest Arachnids; 

 %-,.-. a parasite in human hair-sacs ; X 125. 



They breathe by tracheae. The 



mouth is formed for suction, and they are generally para- 

 sitic. The Mites (Acarus) are among the lowest of Ar- 



