376 APPLIED BIOLOG? 



caused by a parasitic mode of life reminds us of the dodder 

 plant, which loses its roots and leaves, and has only its re- 

 productive structures (flowers) fully developed. Such cases 

 suggest that the reproductive function is the all-important 

 one among animals and plants ; and it certainly is true that 

 many of the most remarkable adaptations are arranged to 

 secure the perpetuation of a species. 



Some of the barnacles which are attached to floating tim- 

 bers are familiarly known as " goose barnacles." The name 

 originated in Great Britain many centuries ago when it was 

 generally believed that a certain kind of wild goose did not 

 develop from eggs, as do ordinary birds ; but that the bar- 

 nacles along the shore changed into small goslings. Hence, 

 they were named " goose barnacles." This interesting myth 

 was long ago exploded when the life-histories of both the 

 goose and the barnacle became known. Even if we did not 

 now know how these two particular animals develop, we would 

 refuse to believe the barnacle-goose story, unless verified by 

 critical investigation, because it is so well known that " like 

 produces like " in the embryonic development of animals and 

 plants. Every one knows that corn plants do not develop 

 from beans or other seeds, and frogs do not come from eggs 

 of fishes, birds, cats, or other kinds of animals. Individuals 

 of each kind of animal or plant must come from egg-cells 

 formed by one of the same kind. 



ARACHNIDS 



317. The Spiders and their Allies : Arachnida. Familiar 

 examples of this group, which is one of the four leading divi- 

 sions or classes of Arthropoda, are spiders, scorpions, mites, 

 ticks, and the king-crab. Any common spider illustrates the 

 most characteristic structure of the animals of this group. 



318. Study of a Spider. (L) Note that the body consists of a 

 cephalo- thorax (head- thorax), and a large abdomen; in this respect 

 compare with crayfish. 



