336 



SENESCENCE AND REJUVENESCENCE 



three species of turbellarian worms, forms related to Planaria; 

 Fig 165 is the non-motile spermatozoon of the nematode worm 

 Ascaris megalocephala; Figs. 166 and 167 show the two forms of 

 spermatozoa found in certain snails; Figs. 168, 169, and 170 are 



155 



154 



I 



156 



\ 



FIGS. 154-157. Male gametes of various animals: Fig. 154, Nereis, 

 an annelid worm (from F. R. Lillie, '12); Fig. 155, Copris, a beetle 

 (from Ballowitz, 'god); Fig. 156, Raja, a fish (from Ballowitz, 'gob); 

 "\_^ Fig. 157, Triton, a salamander (from Ballowitz, 'gob). 



from various species of Crustacea, and Figs. 171 and 172 from 

 arachnids, but Fig. 171 perhaps represents a stage of spermatozoan 

 development rather than the mature form. 



Usually the male gamete in both plants and animals is highly 

 motile, and the course of its development is to a large extent a 



