278 



THE EVOLUTION THEORY 



It is remarkable how different the sperm-cells sometimes are in 

 quite nearly related species of Daphnids, as a glance at Fig. 66 will 

 show; and, on the other hand, how similar they may be in two 

 species which belong to different families, like Bythotrephes longimanus 

 (b), and Daphnella hyalina (c). The last fact may be explained as an 

 adaptation to similar conditions of fertilization. Both species have 

 effective copulatory organs, and their large delicate sperm-cells must 

 immediately adhere when they come into contact with the shell-less 



Fm. 67. Spermatozoa of various animals, after Ballowitz, KOlliker, and vom Rath. 

 I, man. 2, bat (resperugo). 3, pig. 4, rat. 5, buUfincli. 6, newt. 7, skate (Eaja). 

 8, beetle. 9, mole-cricket {Gryllotalpa). 10, freshwater snail {Paiudina). 11, sea- 

 urchin. Much magnified. 



ovum, and penetrate into it by means of amoeboid processes. Con- 

 versely, the difference between sperm-cells of allied species like Sida 

 crystallina (a), Moina rectirostris (e) and M. paradoxa {d) is related to 

 different adaptations to nearly the same conditions of fertilization. 

 In Sida (Fig. 66 a) the large flat sperm-cells, with their fringed ends 

 and their large soft surface, adhere easily to the ova, and the same 

 end is attained in Moina rectirostris by means of stiff radiating 

 processes, while in the nearly related species, Moina paradoxa, the 

 male cell (d) resembles an Australian boomerang and presses in 

 like a wedge between the ova and the wall of the brood-sac. 



In Fig. 67 a small selection of animal male cells is figured, all of 



