THE SCIENCE OF LIFE. 



at the bottom of the sea instead of for swimming, and 

 in the abdomen, or tail, being a mere rudiment which 

 folds into a groove under the enormous thorax. The 

 curious metamorphosis of the Crabs is illustrated in 

 Fig. 126. At first the embryo is a comical-looking ani- 

 mal, with a sort of spiked helmet on its head. It has 

 two large eyes and a well-developed abdomen. It is 

 called a "Zoea," and was formerly described as a distinct 

 genus. After a succession of molts it becomes a per- 

 feet Crab. 



4. ARACHNIDA (arachne, a spider) is a class much re- 

 sembling the Crustaceans, having the body divided into 

 a cephalo-thorax and abdomen. The head carries two, 

 six, or eight eyes, which are not compound bundles of 

 crystal rods covered by a common cornea, as in Crusta- 

 ceans, but separate transparent cones surrounded with 

 pigment. Antennae are only two, if present, and are not 

 used as " feelers," but serve prehension of food. Man- 

 dibles are always present, and in Scorpions the maxillary 

 palps form pincers, or claws, like those of a Ciab. Such 

 claws are called chela, (chele, a claw.) Arachnids are all 

 air-breathers, having spiracles which open into air-sacs, 

 or tracheae. The young of the higher forms undergo no 

 metamorphosis after leaving the egg. The class is di- 

 vided into three orders : Mites, Scorpions, and Spiders. 



I.) Mites are the simplest forms of the class. They 

 have no marked articulations, the head, body, and 

 thorax being in one piece. They have no brain, but a 

 single ganglion in the abdomen. They breathe by tra- 

 cheae. The mouth is formed for suction. Most are 

 parasitic on animals or plants. Mites (Acarus) include 



