54 THE EVOLUTION OF SEX. 



II. Associated Ducts. — It is only in a few animals, like hydra 

 and its allies, that the ovaries and testes are external organs, which 

 have simply to burst to liberate their contents. They are usually of 

 course internal, and thus arises the necessity of some means of com- 

 munication with the outside world. In the simplest cases, the male 

 elements find their way out to the surrounding medium without any 

 specialized mode of exit. They there meet, by chance combined with 

 physical attraction at short range, with the ova, which in the simplest 

 cases again have found their way out in an equally primitive fashion. 

 Thus in the enigmatical parasitic mesozoa (orthonectids, &c), libera- 

 tion of the germs may occur by perforation or by rupture of the 

 excessively simple bodies. In some of the marine worms (for example, 

 Polygordius), the liberation of the ova at least is accompanied by the 

 fatal rupture of the mother organism, a vivid instance of reproductive 

 sacrifice. Even in some of the common nereids, the same uneconom- 

 ical mode of liberation by rupture appears to occur. The forcible 

 rupture may be referred to pressure of the relatively large mass of 

 growing cells which the ovaries often present. 



As high up as backboned animals, the absence of ducts may be 

 traced. Thus among the seasquirts or tunicates, the reproductive 

 organs are frequently ductless, and the same thing is true of some 

 fishes. The sex-cells burst into the body-cavity, and thence find their 

 way to the exterior by apertures. In most cases, where ducts are 

 absent, fertilization of the ova is external, but this is not necessarily so. 

 In sponges, for instance, fertilization is almost always internal. Male 

 elements are washed in by the water-currents, find their way to the 

 ova, and fertilize them in situ. Almost without exception, embryo 

 sponges, not ova, make their way to the exterior. In the higher 

 animals, where definite ducts are present, alike for the inward passage 

 of spermatozoa and the exit of ova or embryos, it ought further to be 

 noticed that the ovaries can hardly ever be said to be in direct con- 

 nection with their ducts. The ova usually burst from the ovary into 

 the body-cavity, whence they are more or less immediately caught up 

 by or forced into the canals, by which they pass outwards. With the 

 testes it is different, for if ducts be present, they are in direct connec- 

 tion with the organs. 



It is enough to state that in the great majority of cases ducts are 

 associated with the essential organs. Those of the male serve for the 

 exit of the spermatozoa, and may be terminally modified as 

 intromittent organs. Those of the female serve either solely for the 

 emission of unfertilized eggs, or for the reception of spermatozoa, and 

 the subsequent exit of fertilized ova or growing embryos. In some 

 worm-types, and in all vertebrates, from amphibians onwards, the 



