THE EVOLUTION OF ORGANIC FORM. 87 



manage to progress slowly, but they depend more particularly on their 

 armor for protection. 



In the Mollusk family another form of reversion from the primi- 

 tive lonsritudinal or three-axed form the conditions of existence ne- 

 cessitate other motive organs. This family of animals, instead of 

 clothing itself in a dermal armor like the Echinoderms, produces a limy 

 covering, a movable house to which it is not anatomically connected, 

 and which principally differs from the stone mansion of the polyp in 

 being movable. Within this house the mollusk preserves his three- 

 axed form ; having no such strong inducement to yield it as has the 

 Echinoderm. But his contact with exterior nature is but a head and 

 foot contact. He therefore develojDS head-limbs tentacular organs 

 while his slow progress is gained by alternate expansions and contrac- 

 tions of a muscular portion of the ventral surface. 



Thus the lower types of animal form are forced, by the necessities 

 of their environment, to evolve certain general anatomical conditions, 

 which, as we shall hereafter see, act as a fatal drag on their subsequent 

 efforts to occupy the higher fields of life. 



The worm type has, from the beginning, a marked advantage over 

 them. The creeping forms of this type would naturally tend to de- 

 velop moving organs at their points of contact with the ground, yield- 

 ing ventral limbs, extended along the body. Breathing organs might 

 appear on the dorsal surface, in contact with the water ; or at the 

 mouth, where inflowing currents would yield the fullest water contact. 



In the swimming w^orms a somewhat different process of limb de- 

 velopment would naturally arise. Here, for the freest degree of mo- 

 tion, some form of fin must replace the limb of the creeping worm. 

 Thei-e is reason to believe that fins first arose as lateral extensions of 

 the flattened body. This general fin under the late theory of limb 

 development in time lost its continuity, and broke up into four sepa- 

 rate sections, whence arose the four limbs of the future Vertebrates. 



The possession of such longitudinal body-limbs or fins gives much 

 greater rapidity of motion to the worm type than is possible to the 

 head-limbed or radiate body-limbed types. As a consequence of their 

 motive facility they remain naked, rapidity of motion and keenness of 

 sense giving them powers of attack and escape not needed by the ten- 

 tacled and armored forms. 



In fact, the advantage of the longitudinal extension is so patent that 

 we find all the lower types making efforts to attain it, and in this man- 

 ner reaching their highest limit of progression. This constitutes the 

 next step in the evolution of animal form, and one which presents some 

 exceedingly curious phases. 



The phases here referred to are not displayed by the mollusks or 

 the Echinoderms. We shall therefore first speak of their simpler mode 

 of attaining their highest development. 



In the mollusk it is attained by a lengthening of the compact body. 



