Modifying Agencies. 143 



it pilots the way. The one at the other end is called " pygidium," which 

 means simply a "little buttock," and might as well be called the tail. 

 It possesses sensitive cirri or feelers which project toward the rear and 

 give warning of hostile demonstrations from that direction. The build- 

 ing of a single body out of a consecutive series of sac-like animals, 

 would require a particular pattern that could be fitted together and 

 made to cooperate. Obviousl} r , those having the U-shaped intestine 

 would not do. 



The segments of a worm of the simpler sort have an alimentary 

 canal running straight through from end to end, each of the composing 

 segments being perforated at each extremity. In the higher grades, 

 much complication and twisting occurs in this organ, and the somites 

 gradually lose their identity. Various viscera may be constructed in 

 some of the segments which do not extend to the others, though serv- 

 ing all. The exit of the alimentary canal, which should be in the ex- 

 treme segment, becomes located further forward, and the tail segment 

 becomes specialized for work. 



The kind of work it first became fitted to do was locomotive. To an 

 animal living in the water, a tail is a vast acquisition. This is proved 

 by the fact that when the land mammals, with good limbs for getting 

 around on land, and with diminished tails, took to the water and 

 adopted an aquatic life, their limbs became modified backward toward 

 the type of fins, and their tails and hind quarters gradually became 

 powerful propellers. As the worm became modified into the vertebrate 

 type, the segmental limbs became eclipsed by the great progress of the 

 tail as the main propeller. The Amphioxus ( Lancelet ), the lowest ver- 

 tebrate now living, has no vestige of bilateral limbs, and those which 

 were afterwards developed in the vertebrate, arose from folds of the 

 skin along the sides, back and belly of the animal, as assistants to the 

 main propeller, the tail. 



The tail of the early fish is straight and tapering to a point ; but in the 

 bony fishes this tail is greatly improved. Width is added to it by the 

 development of a fin on the underside, which at first gives the tail the 

 appearance of being forked unequally, but as this underpart continues 

 to increase, the tail becomes symmetrical. This is accomplished at the 

 expense of much alteration and unseemly distortion of the rear end of 

 the ancient backbone of the old cartilaginous fishes. But new habits 

 of activity required the alteration, and made it. A superiority of the 

 tail as a propeller made locomotion easier and opened the way for a 

 larger energy to go towards improvement in other directions. The 

 head and tail are complemental of each other, and the differentiation of 

 the tail from the rest of the body at one end means also the differentia- 

 tion of the head from the other end. As long as the tail remains an ac- 



