288 COMPARATIVE ZOOLOGY. 



CLASS III. 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. y 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 may be 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 ffan- 



Fio. 258. A Mite (Demodex follicnlo- < . 



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



a parasite in human hair-sacs ; X 12S, rp. , , . . . 



1 hey breathe by tracheae. The 



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

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

 ticulates. The body is soft and minute. The Ticks 

 (Ixodes) have a leathery skin, and are sometimes half an 

 inch long. The mouth is furnished with a beak for pierc- 

 ing the animal it infests. 



2. Pedipalpi, or Scorpions, characterized by very large 

 maxillary palpi ending in forceps, and a prolonged, joint- 

 ed abdomen. The nervous and circulatory systems are 

 more highly organized than those of Spiders; but the 



