ARTHROPODA. 



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 antenna, if 

 present, are only two, and these are not u 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 . 258 ._ A Mite {Bemodex ^ lliculo . 

 glion lodged in the abdomen. oue of the lowest Arachnids; 



~L a parasite in humau 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- 



