154 



ANIMAL PHYSIOLOGY. 



[LESSON 43. 



FIG. 251. 



passes through the narrow pedicle of the abdomen, it is contracted ; 

 it slightly expands in its straight course (/) along that cavity, then 

 contracts and forms two short convolutions (g), and communicates 

 with a large globular coecuiu (h) from which the last short intestine 

 arises. Four bile ducts (i, i) open into each 

 side of the straight portion of the intestine. 

 Two long, slender tubes (k, k), constituting the 

 kidney, communicate with the commencement 

 of the co3cum. 



681. There is a simple and easy mode of 

 finding the class to which the articulata belong, 

 at a glance : thus, if the body be divided into 

 three chief portions, it is sure to possess just 

 three pairs of legs ; therefore the creature is 

 an insect. But if four pairs of legs be 

 present, this character is invariably found as- 

 sociated with another of great importance; 

 that is to say, the body will have only two 

 divisions,' the head and the chest being sol- 

 dered together to form the cephalo-thorax (head, 

 ion ' Spid( chest), and the remainder constitutes the ab- 

 domen, or body. Here, then, we have the class Arachnida typified, 

 and the specimen is either a Spider (the Greek name of the class), a 

 Scorpion, or a Mite. If Jive, six, or more pairs of legs be present, 

 again we find the two divisions of the body indicating the same parts 

 as those already described (cephalo-thorax, and body), and these 

 form the characteristics of the Crustaceous animals. 



682. To descend still lower in the scale : if the specimen pre- 

 sented to our notice have a variable number of distinct rings, or seg- 

 ments, any one of which is like all the rest, and the legs numbering 

 twenty-two, or more pairs, the specimen belongs to the class Myrio- 

 poda many feet to which the Centipedes, lulidce, Wood-lice, &c., 

 belong. Size has nothing to do with it ; the Mites (many of them 

 microscopical specks) having the required characteristics, as much 

 belong to the Arachnida as though they were as large as a Scorpion ; 

 and so of all the rest. 



