Homology in Plants 93 



ages. The head and thorax are fused together, in contrast to 

 the distinct head and thorax of insects (Fig. 24) . Close ex- 

 amination, however, reveals a faint suggestion of segmenta- 

 tion in the thorax, with a pair of appendages correspond- 

 ing to each segment. It is impossible to find distinct 

 segments in the head region, but the series of appendages is 

 continued in the form of various mouth-parts or " food 

 graspers," convenient substitutes for forks and spoons and 

 pushers. If we examine one of the abdominal legs, we find 

 that each consists of a basal joint with two branches. The 

 number of joints on one of the walking legs is easily counted: 

 and the same number appears on each of the others, includ- 

 ing the large nippers. The organs about the mouth, including 

 the " jaws," which work from side to side, and the " lips " 

 are all modifications of the same " leg " pattern. In some 

 of these mouth-legs, the outer branches are sensory; in others 

 they push food into the mouth; and in still others they main- 

 tain the current of water which supplies the gills with fresh 

 oxygen and carries away the carbon dioxid. The abdominal 

 legs assist somewhat in swimming, but in the male one pair 

 is modified as an organ for transferring the sperms. 



Among the insects, which show perhaps the greatest 

 range of specialization found in any class of animals, the 

 different orders have different kinds of mouths, adapted to 

 different feeding habits (Fig. 25). Yet in every case the 

 fundamental plan is exactly the same, and traceable in detail 

 to the corresponding parts of less specialized arthropods, and 

 to a primitive ** leg " pattern. 



Homology in slants 



In plants as well as in animals we find that fundamen- 

 tally identical structures are applied to a great variety of 

 functions. In the grape a leaf is " modified " into a climb- 

 ing organ, or tendril. In the black locust, a pair of stipules, 

 which are leaflike in roses and willows, appear as sharp thorns. 

 In the flowers of the vast array of seed-bearing plants, 



