184 BIOLOGY FOR BEGINNERS 



remarkably division of labor may be carried out, and second that 

 we have here the modification of one kind of organ for many uses. 

 The tabulation will show how many and how varied are the func- 

 tions performed. It will also be seen that these various organs are 

 developed from a simple kind of appendage (the swimmeret). By 

 the addition and modification of segments, organs have been de- 

 veloped as widely different as the large claws and the antennae. 



Homology. When we find organs (either in the same or different 

 animals) which were developed from the same part, that is, whose 

 origin and structure are similar, we call them homologous organs. 

 Thus we may say that the antennae and claws of the crayfish are 

 homologous to the swimmerets, or that our arm is homologous to 

 the foreleg of a horse, even though the functions are so different. 

 This word is the mate to analogous which meant similar in function. 

 We might say that the gills of the crayfish and the lungs of man 

 are analogous, because they both perform the function of respira- 

 tion but we cannot say they are homologous, since the gills are de- 

 veloped from the legs, while the lungs are outgrowths of the throat. 



Internal Structure. Internally, also, there is a considerable de- 

 gree of specialization. The digestive system and its glands occupy 

 a large part of the cephalo-thorax, there being three sets of kvth 

 in the stomach, to complete the chewing of the food which was 

 begun by the jaws. A well-developed circulatory system and a 

 complicated heart mark an advance along this line. The e\m i tory 

 and reproductive organs are present and fairly developed. The 

 nervous system, though similar, is much more specialized than in 

 the worms. The senses of touch and smell, located in the antennae, 

 are probably acute. The eyes are on movable stalks and are com- 

 pound, each consisting of numerous lenses, but the sight is prob- 

 ably not keen. The ears are located at the base of the antennae 

 but neither hearing nor taste seem to be especially developed. 



While these sense organs do not seem very efficient, yet enor- 

 mous advance can be seen when they are compared with the earth- 

 worm with no organs of special sense at all. The worm probably 

 feels only touch and vibration sensations through the body wall, 

 with a possibility of taste and heat or light sensations in the region 



