298 SEXUAL DIMORPHISM 



theory. Up to a length of 15 mm. the worms of this species 

 are all similar, of the ordinary form, and sexually immature. 

 From these immature young worms develop several different 

 mature forms. Some individuals become sexually mature 

 without changing their form, only growing larger. Such 

 individuals are 15 to 30 mm. long. They live in tubes 

 formed by excretion from the skin, and in the tubes fertilised 

 eggs were found. At Messina Von Westinghausen found 

 these eggs from April to the end of July, at Naples from 

 December onwards. Other individuals, without growing 

 much larger, are transformed into what is called the small 

 Heteronereis form, which appear in an active pelagic condition 

 in February and March, and are sexually mature. These 

 Heteronereids are from 20 to 40 mm. in length, and have 

 65 to 75 segments. The eggs in the two cases are quite 

 different, and there is a corresponding difference in the 

 development. The eggs of the mature Nereis form have 

 more yolk and develop directly without metamorphosis, 

 those of the Heteronereis are laid in gelatinous clumps at 

 the surface of the water, contain little yolk, and give rise to 

 a trochophore larva which by metamorphosis develops into 

 the young worm. Again, a third set of the immature 

 individuals grows to from 55 to 65 mm. long without 

 becoming sexually mature, and then are transformed into 

 the large Heteronereis form which is found sexually mature 

 in August. One of the most remarkable facts in this 

 complicated history is, that the large Heteronereis form 

 is not pelagic, but lays eggs with a small quantity of yolk, 

 in its tubes. The development of an immature individual 

 may thus take either of three distinct courses : it may 

 become mature without metamorphosis, it may be transformed 

 into a pelagic Heteronereis and produce pelagic ova, or it may 

 be transformed at a much larger size into a non-pelagic 

 Heteronereis, which produces non-pelagic ova. It is entirely 



