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 



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, '^od); Fig. 156, Raja, a fish (from Ballowitz, '906); 

 Fig. 157, Triton, a salamander (from Ballowitz, '906). 



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 



