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'HE EVOLUTION OF ORGANIC FORM. 89 



this line of animal evolution has reached its ultimate at a much lower 

 level than that attained by the Mollusca. 



But, by this review of what we may, in a figurative sense, call Na- 

 ture's failures in animal evolution, we begin to perceive the requisites 

 to success. The retrograde forms, after again developing into the 

 lengthened type, are constitutionally restricted from gaining certain 

 structural advantages which are primitive possessions of other types. 

 These advantages we may classify as body-limbs, adapted to walk- 

 ing or swimming ; and an articulated body, capable of a flexibility not 

 possible to the compact, single-sectioned animals. All the other ani- 

 mal types, besides those we have considered, have made an effort to 

 attain this articulated structure, sometimes by a very curious process. 

 The success attained in this effort is closely dependent upon the primi- 

 tive structure of the articulated animal, which has placed impassable 

 restrictions in the path of some types. 



In the polyps and in the articulates the end seems to have been 

 attained by the linking together of a colony of animals, forming a 

 structure, originally compound, which has become simjjle by a division 

 of functions between the successive sections. 



In the Vertebrata alone has it been attained by the articulation of 

 an originally single animal. The vei'tebrates thus possess special 

 structural advantages denied to the other articulated forms, the com- 

 pound origin of these latter curiously limiting their powers of evolu- 

 tion. 



In this merging of societies into single animals, Nature presents us 

 instances of every step of the process, from those in which individual- 

 ity remains intact, to those in which it is subordinated to the require- 

 ments of the compound animal. 



A first step in the process is displayed by the Tunicate mollusks. 

 The Salpa one form of these shell-less creatures is a free-moving 

 animal, progressing by the aid of water, which is drawn into one end 

 of its straight intestine and expelled at the other. They exist in two 

 conditions, the single and the compound. In the latter they unite into 

 long chains, not organically connected, but apparently adhering by 

 little suckers. 



This primitive combination seems assumed for one advantage only, 

 that of aiding their motion. The animals in the chain conti-act and 

 expand simultaneously, the whole chain moving like one lengthened 

 animal. 



The same end is achieved in a still more curious way in the Pyro- 

 soma, another of the Tunicata. These little creatures so group them- 

 selves as to form a hollow tube, open at one end and closed at the 

 other. The minute animals which compose the walls of this tube have 

 one gill-opening extended outward, the other inward. Thus they draw 

 water from outside and discharge it into the interior of the tube. 

 This being closed at one end, the water is necessarily driven from the 



