2 FIELD ZOOLOGY. 



animals, many of them highly differentiated. Under the 

 second class, he may establish such branches as the 

 Coelenterata, animals with a continuous body cavity, this 

 cavity having but a single opening, which serves both as 

 mouth and as anus; Echinodermata, animals of radiate 

 structure, no backbone, and the body surface beset with 

 spines (from e chinos, a hedgehog, and dermos, skin) ; 

 Mollusca, including such animals as snails and clams; 

 Arthropoda, animals with organs of locomotion jointed in 

 successive segments, literally joint-footed (from arthron, 

 meaning joint or articulation, and poda, meaning organs of 

 locomotion) ; and Chordata, animals having a nervous 

 cord, or a backbone, or both. The last-named branch 

 includes such animals as the sea squirts with only a ner- 

 vous cord; and the fishes, the frogs, the reptiles, the birds, 

 and the mammals, with a vertebrate skeleton. 



This work confines itself to the study of portions of 

 two of these branches the Arthropoda; and the Aves, or 

 birds, under the Chordata. Under the branch Arthropoda 

 are found insects, scorpions, mites and ticks, millipeds 

 and centipeds, spiders, and the large class of the crusta- 

 ceans. The crustaceans will be left for consideration else- 

 where, and we shall confine our study to the insects with 

 their near relatives, and birds. 



The class Insecta includes only the insects, and it is 

 itself divided according to the similarities and dissimi- 

 larities of the many kinds of insects. 



The name insect is applied to such animals as have 

 the body cut into successive segments. These segments 

 are for the most part grouped so as to form three general 

 regions, the head, the thorax, and the abdomen. The 

 whole body may be composed of distinct and similar 

 segments, as in the caterpillar; or may be greatly modified 



