130 SENESCENCE AND REJUVENESCENCE 



species the posterior fission-piece undergoes a considerable increase ; 

 the anterior, except when very large or heavily fed, exhibits a 

 slight increase in susceptibility. In other words, agamic repro- 

 duction brings about a greater or less degree of rejuvenescence. 



AGAMIC REPRODUCTION AND REJUVENESCENCE IN Planarid Velata 



Planaria velata (Fig. 8), a flatworm found very commonly in 

 temporary pools and ditches as well as sometimes in permanent 

 bodies of water, is another species in which only agamic or asexual 

 reproduction has been observed during some thirteen years. The 

 asexual cycle of this species and its relation to senescence and re- 

 juvenescence have been considered at length elsewhere (Child, 

 '136, '14), and only the more important points need be reviewed here. 



Agamic reproduction in this species is a process of fragmenta- 

 tion which occurs only at the end of the growth period. The 

 animals appear early in spring, chiefly in temporary pools and 

 ditches in which dead leaves have accumulated. When they first 

 appear they are only two or three millimeters in length, very active, 

 and to all appearances young in every respect. They grow rapidly 

 and become deeply pigmented, but the rate of growth gradually 

 decreases, and at the end of three or four weeks, when they have 

 attained a length of about fifteen millimeters, they cease to feed, 

 become lighter in color, their motor activity undergoes a distinct 

 and progressive decrease, and the pharynx undergoes complete 

 disintegration. Within a few days after these changes fragmenta- 

 tion begins at the posterior end of the body. The process of 

 fragmentation resembles in certain respects the process of fission 

 in P. dorotocephala, described in the first section of this chapter. 

 As in that species, the act of separation is accomplished by attach- 

 ment of the posterior end to the substratum while the animal is 

 creeping, with the result that a small piece tears off and is left behind. 

 But in P. velata the process may be repeated frequently in the course 

 of a few hours and the fragments vary widely in size. In P. velata, 

 as in P. dorotocephala, fragmentation is undoubtedly the result of 

 physiological isolation and independent motor reaction of the 

 posterior end of the body, but, instead of occurring periodically 

 during the life of the animal, it does not occur until senescence is 



