'262 LECTUKES ON BIOLOGY 



ocean and develop new colonies far away from home. They 

 serve, therefore, prominently as a means of distributing the 

 species and opening up new regions for the Hydrazoa to colonize. 

 The great importance of this fact to the preservation of the 

 species, the advantage which they possess as a result of this 

 arrangement in the struggle for existence over other fixed 

 organisms which have no free-living sex-animals or similar 

 arrangements for the distribution of their germ-cells and are 

 therefore restricted to their narrowly defined domicile, is obvious. 

 We are probably right in assuming that the medusae owe their 

 origin to this very need. Phylogenetically the polyps represent, 

 therefore, the older and lower stage of development from which 

 the free-moving sexual form was gradually evolved. For how- 

 ever much the medusa may differ in its appearance from the 

 polyp, accurate comparison shows that it is nothing else but a 

 more highly differentiated polyp which has become adapted to 

 a free mode of life. 



It must strike us as very remarkable that some species of 

 Hydrozoa have again abandoned this advantage. It is true that the 

 PlumularidcB and Sertularidce still produce a medusa-generation, 

 but the sex-animals are incomplete and no longer adapted 

 to a free-swimming life. Like the polyps, they are fixed to the 

 stem. The sex-animals have thus become sex-organs. 



The degree of degeneration can vary greatly. In some species 

 only the mouth-aperture, tentacles, and the strong muscular 

 velum is lost, but the protective bell with its most important 

 organs and the mouth-stalk remain. In others the form of the 

 sex-organs hardly permits us even to suspect that they have 

 originated from medusae, and that once they were independent 

 animals. Only the degenerate stomach and the sexual organs 

 have remained, being surrounded by the sparse remainders of the 

 bell as by a thin mantle. 



It is necessary to mention here the alternation of generations 

 in some of the tapeworms. While the larvae of most tape- 

 worms produce only one sex-animal, in the cysticerci of Tcenia 

 ccenurus, the cause of the deadly "gid " of sheep, and of T. echino- 

 coccus, one of the smallest, but probably also one of the most 

 dangerous tapeworms, there are produced by inner budding 

 several, sometimes numerous, tapeworm-heads from which 



