176 PHYSIOLOGY 



developed and open to the outside through ducts that are 

 probably modified excretory organs. The development 

 of unfertilized eggs occurs in bees and a few other forms. 

 In aquatic forms eggs and sperm meet in the water; 

 but in aerial forms union of the reproductive cells takes 

 place in the duct of the female. 



Appendages. — The appendages are primarily loco- 

 motor in function, but they may be modified into sense 

 organs, into chewing jaws or into false feet. The 

 false feet serve as gills, as supports for gills, as places 

 for the attachment of eggs, as organs for the transfer of 

 sperm, or as swimming or creeping organs. The head 

 bears the antennce? or touch organs, the jaws and maxil- 

 lipeds; the thorax, the true feet ; and the abdomen bears 

 the false feet or it lacks appendages. 



Classification. — The Arthropoda are divided into four 

 classes: the Crustacea, the acerata, the insecta, and the 

 myriapoda. 



The Crustacea. — In the Crustacea, gills are present, 

 the feet are two-branched, and the reproductive ducts 

 open near the middle of the body. They are divided into 

 two classes, the first of which includes very small forms 

 mostly microscopic whose bodies vary in the number of 

 segments and in their appearance. The others are larger 

 and their bodies consist of twenty segments. The best 

 known examples are the shrimps, lobsters and crabs so 

 much esteemed for food. Most of them are marine, but 

 some live in fresh water and some live on land. 



Locomotion of the Lobster. — The study of the Lob- 

 ster (Fig. 98) is interesting because it so well illus- 

 trates the characteristics of the class and because it is 



