GENERAL ZOOLOGY 



true arthropods. Fossil remains that are unmistakably onvchophoran are 

 known from the middle Cambrian period, and the external features of 

 Peripalus show remarkably few alterations when compared with those of the 

 ancient forms. Peripatus should not be regarded as a "missing link" between 

 annelids and insects (cf. p. 630). Peripatus itself and onychophores generally 

 are in their own way highly specialized evolutionary end forms, and such end 

 forms cannot logically be considered as links between different phyla. In 

 addition, it would be surprising indeed if insects, the most advanced class of 

 modern arthropods, could be traced directly to annelid-like ancestors. 



The Arthropod Body Plan and Its Evolution 



The fundamental similarities in general organization between annelids and 

 the arthropod-onychophore groups indicate that a common ancestry lies be- 

 hind all these types. If we look for factors significant in determining the 

 different evolutionary directions taken by annelids and arthropods, leading 

 to the marked differences between their modern representatives, one factor 

 stands out pre-eminently — the development of the hardened exoskeleton in 

 the arthropod stem. We may assume that somewhere in the evolutionary 

 history of arthropods, after the divergence of the ancient onychophores but 

 antedating the appearance of chelicerates and mandibulates, the soft cuticle 

 characteristic of the worm-like ancestor began to be replaced bv a more rigid, 

 unyielding, protective coat. Correlated with this development, several other 

 changes must have occurred in the ancient groups. The problem of growth 

 was eventually solved, in a manner of speaking, by perfection of the complex 

 process of molting, with its implication of an endocrine system to integrate 

 the activities of all parts of the body involved. The locomotion characteris- 

 tic of the annelids, either by serpentine undulations or by reciprocal changes 

 in the length and diameter of the body, could no longer be effected; the 

 rigidly encased body could no longer bend or change its shape, except as 

 flexible, intersegmental joints could be by chance developed and perpetuated 

 through natural selection. Correlated with this event must have been changes 

 in the musculature, the formerly extensive sheets of circular and longitudinal 

 muscles being replaced by specifically acting groups of flexors and extensors, 

 operating to bend one part of the body in relation to another. Also involved 

 in this change in locomotion was the development of segmental appendages, 

 with definite joints and muscle groups for their manipulation. With the 

 development of the undifferentiated, generalized series of jointed appendages, 

 capable of specific actions, it became possible for regional specialization to 

 begin. Certain of the anterior segments, we may imagine, became incor- 

 porated into the primitive head, and their appendages were modified until 

 they formed sense organs and mouth parts. Other segmental appendages 

 in the long series must have become variously specialized as swimming 

 paddles, chelipeds, and walking legs. Along with regional specialization, 



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