ALTERNATION OF GENERATIONS 1G9 



fertilized egg of a jelly-fish does not produce a jclly-fisli but 

 a hydroid. Thus we have here an alternation of (Tcnerations 

 — a sexual generation (the jelly-fish) producing sexually, by 

 ova and spermatozoa, an asexual generation (the hydroid). 

 This hydroid in its turn produces asexually, by budding, a 

 jelly-fish again. 



Such alternation of generations is generally associated with 

 the necessity of spreading the species, and it is in the animal 

 kingdom as a rule confined to animals in which one generation 

 is fixed, sessile, or to animals in which one generation is 

 parasitic, such as the liver-fluke and the tape-worm. In the 

 latter class the chance of finding the right host is very small. 

 Hence everything is done to increase the number of repro- 

 ductive cells: in certain tape- worms, for instance, the fer- 

 tilized egg produces a larva which by internal, asexual, budding 

 will produce dozens more larvae, and each of these by internal 

 budding may again produce scores of offspring, all of whom 

 may, if lucky, grow up into adult sexual tape-worms. Then 

 again, the product of a single fertilized egg in a liver-fluke 

 ends in hundreds of thousands of larvae, and the chance of 

 one of them hitting the right host is thus considerably in- 

 creased. 



But, as we have seen, alternation of generations in the 

 animal world does not reach very high up the scale. It is 

 common in certain water worms; but it does not exist in the 

 higher Crustacea, or in mollusca; neither is it found amongst 

 spiders or true vertebrates, nor in many of the other smaller 

 and less conspicuous groups. It attains its highest point in 

 the AsciDiANS, where certain free-swimming forms, such as 

 Salpa, have an alternation of generations. The Ascidians 

 come at the very bottom of the vertebrate stem. 



In plants, however, alternation of generations can be 

 traced, although greatly masked, up to the very highest forms 

 of vascular flowering plants. 



In the simplest of the brown seaweeds we find i^lants which 

 consist of branched cellular threads — filaments; these repro- 

 duce by both the methods which have been described; some- 

 times they form in their ceHs zoospores which germinate 

 directly into new plants and sometimes they produce gametes 



