ANIMAL AND PLANT REPRODUCTION 23 



by group, for example unisexual land or fresh-water 

 worms with their bisexual marine cousins, he finds the for- 

 mer to be the more complex, particularly as to their sex 

 organs. The fact that the sponges are hermaphroditic 

 might be considered as weighing against this argument, 

 but it is not without the bounds of probability that the 

 sponges are further along in specialization than is gen- 

 erally admitted, for to find the substance nearest chem- 

 ically to the so-called skeleton of the sponges, one must 

 search among the arthropods the product of the spin- 

 ning glands of certain spiders and insects. 



Hermaphroditism, pure and simple, however, was not 

 a success. Only a few degenerate forms retained self- 

 fertilization and persisted. Among them may be men- 

 tioned the tapeworms, certain crustaceans (Sacculina) 

 parasitic on crabs, and the colonial forms, bryozoans and 

 tunicates, the latter being perhaps the most degenerate of 

 all animals since they are wholly unrecognizable as rela- 

 tives of the vertebrates except at one short stage of their 

 life history. In most of the hermaphroditic types new 

 characteristics appeared which enabled them to exercise 

 one of the important functions of bisexuality, cross-fer- 

 tilization, without giving up the obvious energy conserva- 

 tion attainable through the production of both sex cells 

 in a single individual. 



In nearly all of these forms, this was made possible by 

 the development of the eggs and of the sperm at different 

 times. In a few isolated cases among the turbellarians 

 and the tunicates the eggs develop first and then the 

 sperm; the animal is first a female and later a male (pro- 

 togyny). But in a greater number of species, the indi- 

 vidual is first a male and afterwards a female (pro- 



