GENERATIVE ORGANS. 373 



in front of the last segment of the asexual form, and these segments, 

 after the formation of a head, constitute a new individual. As this 

 process is repeated, a chain of connected individuals is formed, and 

 these, as soon as they are separated, represent the sexual individuals. 

 Among the freshwater Naidce, in Chcetogaster, a regular and continued 

 budding in the long axis leads to the formation of chains, consisting of 

 not less than 12 to 16 zooids, each having only four segments, while 

 the sexual individuals consist of a greater number of segments. A 

 similar process occurs in the mode of reproduction observed by O. 

 Fr. Miiller in Nais proboscidea, from the last segment of which a 

 new zooid is produced. Both generations of Nais, however, be s me 

 sexually mature. 



[For a more complete account of the asexual reproduction of Chsetopoda, 

 vide Balfour, "Comparative Embryology," vol. i., pp. 283, 284.] 



The Chcetopoda are, with 

 the exception of the her- 

 maphrodite Oligochceta and 

 certain Serpulidce (e.g., Spi- 

 rorbis spirillum, Protula 

 Dysteri) of separate sexes. 

 Male and female individuals 

 seem occasionally so strikingly 



different in the Structure of FIG. 303. A parapodium of Tomopterii with a 



their organs of sense and lo- ass f va and one free ovum < after c - 



Gegenbaur). 



comotion that they have even 



been taken for species of distinct genera. Besides the above- 

 mentioned Sacconereis and Polybostrichus, the asexual generation of 

 which is Autolytus, a similar sexual dimorphism has been shown by 

 Malnigren for Heteronereis, a genus of the Lycoridce, in which the 

 males and females differ both in external form and in the number 

 of their segments. A remarkable case of heterogamy is also 

 afforded by this genus, in that a generation of smaller animals 

 swimming upon the surface alternates with a generation of arger 

 forms living upon the bottom. 



The generative apparatus of the Oligoc/iceta is very highly deve- 

 loped. The ovaries and testes lie in definite segments, and empty 

 their contents by dehiscence of their walls into the body cavity. 

 Special generative ducts often co-exist with segmental organs in 

 the same segment (0. terricolce), while in other cases the segmen- 

 tal organs are wanting in the generative segments (0. limicolce}. In 



