86 THE POPULAR SCIENCE MONTHLY. 



one being in contact with the ground, the other with water or air. The 

 intermediate surface is affected in but one manner. It being every- 

 where similarly influenced, its whole development is similar, and a ra- 

 dial or two-axed form results. 



In the lengthened forms the longitudinal or intestinal axis naturally 

 becomes horizontal. The two extremities of this axis develop into 

 mouth and vent. But the intermediate portions are also differently 

 conditioned. Vertically a lower face is in contact with the ground, an 

 upper face with water or air. Thus ventral and dorsal surfaces are 

 produced. Transversely, the oj^posite sides are similarly affected, and 

 develop similarly. Gravitation also tends to produce a shortening of 

 the vertical and a widening of the transverse axis. Thus the three- 

 axed animal appears, a lengthened form, with mouth-opening at the 

 anterior-moving extremity, with diverse dorsal and ventral surfaces, 

 with similar lateral surfaces, and with a tendency to become flattened 

 vertically. 



In this lengthened, or worm type, appears an animal form more 

 highly conditioned than any possible two-axed form, and capable of 

 far higher development. It is much the best adapted for rapid move- 

 ment, its long, narrow shape being well calculated to overcome the 

 frictional resistance of water or au' ; while it is capable of a flexibility 

 not possible to the compact types of animal form. 



Consequently, from the primitive types of animal form we have so 

 far considered, we find two general lines of development. The first of 

 these is a tendency in the compact types to become lengthened in form, 

 to lose their protective armor, and to assume the free-moving condition 

 their most advanced genera being thus constituted. On the other 

 hand, a retrograding tendency shows itself in certain sections of the 

 lengthened animal type, compact forms ajipearing. And, significantly, 

 these envelop themselves in shelly or horny armor for protection, be- 

 come sluggish in motion, and fail to develop the acute sensitiveness and 

 other advanced powers of the naked worms. 



Such being the primitive and secondary form-evolving tendencies, 

 the production of organs of motion is in strict accordance therewith. 

 In the radiated polyps the limbs appear as head-organs. These are 

 so ill-adapted to the production of free motion, that the solitary Polyps 

 have not developed into this condition, except in the case of the Me- 

 dusae or jelly-fish. And these in no instance seek to swim by aid of 

 their arms, a slow movement being gained by umbrella-like contrac- 

 tions of their radiate body disk. 



In the Echinoderm family one of the reversions from the longitu- 

 dinal type the shortening of the intestinal axis has brought it into 

 such close relations with the radiate type that the difference is only 

 clearly distinguishable in its embryo stage of existence. Its intestinal 

 axis has become vertical, and it has gained radiated limbs not head, 

 but side limbs. By the aid of these the free-moving Echinoderms 



