KINETOGENESIS. 269 



of existing forms were produced. We can believe, 

 however, that, as in Vertebrata, there has been a 

 gradual elimination of less important segments of the 

 limbs, until the highest mechanical efficiency was at- 

 tained. We well know how the segments of the head 

 and body have been modified by fusion, etc. 



Prof. B. L. Sharp has shown the mechanical con- 

 ditions of segmentation in Arthropoda as follows: 1 



"It occurred to me that if the theory [of kineto- 

 genesis] had a general application, some additional 

 proofs could be shown to exist among the inverte- 

 brates, where we have the action of muscular force 

 upon hard and resisting parts of the skeleton. Those 

 which present the best study for this purpose appear 

 to be the crustaceans, where we find an immense va- 

 riety of articulations in the body and in the limbs; 

 highly complicated locked joints, others allowing mo- 

 tion in but one plane, as well as loose joints, where 

 the hard parts scarcely come in contact with one 

 another, and cases of degeneration of the hard parts, 

 leading to total disappearance of a previously existing 

 joint. 



"In the Anneiides, from which, there is no doubt, 

 the arthropod branch sprang, we find no deposit of 

 inorganic salts in the epidermis. The outer layer of 

 the body is generally of a horn-like character, adher- 

 ing closely to the secretive cells of the epidermis, very 

 flexible, and thrown into folds by the vermicular mo- 

 tion of its possessor. In the leeches the body consists 

 of a flexible cylinder, made up of two sets of muscles, 

 an outer longitudinal cylinder and an inner cylinder of 

 circular fibers* the contraction of which causes the 

 animal to increase in length, while shortening is ef- 



1 American Naturalist, 1893, p. 89. 



