770 LIFE : OUTLINES OF GENERAL BIOLOGY 



rise directly to other medusoids without the interpolation of a fixed 

 polypoid or hydroid stage. This is part of the argument that leads 

 to the conclusion that the ancestors of both medusoids and medusae 

 were free-swimmers; and that the fixed, vegetative, asexual phase 

 has been interpolated secondarily. Yet some naturalists read the 

 story in a precisely opposite way, and regard the fixed stages as the 

 more primitive. 



This seems to us improbable ; but the uncertainty does not affect 

 our general suggestion, that alternation of generations is to be 

 regarded as a special outcome of the plasticity of particular arcs in 

 the life-curve, and of the tendency to interpolate larval stages to 

 meet certain difficulties. It should also be noted that larvae some- 



FiG. 127. 



Diagram of Alternation of Generations, i. In a Hydromedusoid life-history. 

 A fertilised ovum (OV) develops into a free-swimming larva which 

 settles down and becomes a polyp, which asexually buds off (A) a hydroid 

 colony. From this by budding are produced free-swimming medusoids 

 (S), which become males and females, from which arise the fertilised 

 ova, starting a fresh cycle. 2. In a liver-fluke life-history. A fertilised 

 ovum (OV) develops into an asexual larva, which produces by means 

 of spore-cells (R) a succession of asexual larvae. But eventually the last 

 larval form develops into a sexual fluke (S), producing self -fertilised ova. 



times become precociously reproductive — a peculiarity also pointing 

 the way to alternation of generations. 



From a sheep infected with liver-rot the microscopic developing 

 embryos of the liver-fluke drop on to the wet pasture. Out of the 

 eggshell comes a ciliated free-swimming larva which may find its 

 way into a small water-snail. There it gives rise by means of un- 

 fertilised egg-cells, like spore-ceUs, to another kind of larva (redia), 

 which immediately proceeds to do the same. Eventually there are 

 formed little tailed larval flukes (cercarise), which leave the snail 

 and the water, and encyst on blades of grass. If one of these minute 

 white specks be swallowed by a grazing sheep, the young creature 

 passes from the food-canal to the liver, and there grows rapidly 

 into the hermaphrodite fluke. In this case the rediae and cercariae are 

 produced not asexuaUy, in the strict sense, but from germ-cells 

 which do not require fertilisation and might be called spores. 



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