146 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



paddle-like form may make their appearance. These modifications of 

 the parapodia have to do with the fact that the Nereis at this stage gives 

 up its bottom-frequenting habit and swims about freely so as to distribute 

 the gametes over a wider area. With the assumption of the pelagic 

 habit there comes about another modification very usual in pelagic 

 animals namely a great increase in the size of the eyes. 



In an allied family of Polychaetes, the Syllidae, to which a number 



K 



Fig. 70. 

 Diagram illustrating sexual phenomena in the Syllidae 

 (from Benham, Cambridge Natural History). I, Heterosyllid ; 

 II, Syllis hamata ; III. and IV, Autolytus ; V, ^. eduiardsH, 

 Myrianida. A , Asexual individual ; B^ C, D, E, sexual 

 individuals, z Early stage in development of a sexual 

 individual. 



of common marine worms belong, there occur similar modifications, 

 carried in some cases much further than in Nereis. In the simplest 

 case (Fig. 70, I) what takes place is very much the same as in Nereis, 

 a heterosyllid condition being assumed in which the hinder part of the 

 body containing the gametes takes on an appearance markedly different 

 from that of the front part. In exceptional species of Syllis (S. hamata) 

 after the heterosyllid condition has been reached there makes its appear- 



