304 THE STUDY OF ANIMAL LIFE CHAP. 



is then said to be hermaphrodite. Some sponges and 

 stinging animals, many "worms," e.g. earthworm and 

 leech, barnacles and acorn-shells among Crustaceans, one 

 of the edible oysters, the snail, and many other Molluscs, 

 the sea-squirts, and the hagfish, are all hermaphrodite. 

 But it should be noted that the organs in which ova and 

 spermatozoa are produced are in most cases separate, 

 that the two kinds of cells are usually formed at different 

 times, and that the fertilisation of ova by spermatozoa 

 from the same animal (e.g. in liver-fluke) is very rare. 

 In some cases hermaphroditism may have arisen as a 

 secondary complication, derived from, not antecedent to, 

 the ordinary (dioecious) condition. In other cases a 

 hermaphrodite state of periodic maleness and femaleness 

 may be primitive. Except in Tunicates, a few fishes and 

 amphibians, and casual abnormalities, hermaphroditism 

 does not occur among the backboned animals. 



(b) Parthenogenesis seems to be a specialised form of 

 sexual reproduction in which the ova produced by female 

 organisms develop without being fertilised by male cells. 

 Thus " the drones have a mother but no father," for they 

 develop from ova which are not fertilised. In some 

 rotifers the males have never been found, and yet the 

 fertility of the females is very great ; in many small 

 crustaceans (" water-fleas ") the males seem to die off 

 and are unrepresented for long periods ; in the aphides 

 males may be absent all the summer (or in a greenhouse 

 for years) without affecting the rapid succession of female 

 generations. 



In a few multicellular animals there is a kind of multi- 

 plication by spore-cells, which require no fertilisation 

 but are hardly to be regarded as on the level of egg-cells. 

 They are well-illustrated by the cells in the sporocyst of 

 the liver-fluke (p. 235) which give rise to redise, or to the 

 cells in the redise which give rise to more redise or to 

 cercarise. Perhaps these spore-cells represent a harking- 

 back to a very primitive mode of multiplication by means 

 of germ-cells which were not as yet differentiated as sex- 

 cells. 



(c) Alternation of Generations. A fixed asexual zoo- 



