Summary. There were originally 23 species of Syllids known in the Gulf of Naples, this number now is extended to 59 including the 5 species and 4 subspecies as described by the Author. There are 22 new species for the Gulf, 2 of which are new for the Mediterranean. For a clear idea of the distribution of Syllids in the various environments, six types of bottoms have been considered, namely: 1) Submerged reef (Depth m. O - 1 O); 2) Sandy shores (Depth m. O - 10); 3) Meadows of Posidonia (Depth m. 1 - 30); 4) 1st Corallin zone (Depth m. 20 - 50); 5) Bottoms with Peyssonnelia (Depth m. 50 - 70); 6) 2nd Corallin zone (Depth m. 50 - 120). In the muddy bottoms Syllids have not yet been collected. Sandy shores excepted where only specialized forms live, Syllids in the other environments sometimes overlap into adjacent bottom types according to the adaptivity of each species. It is not possible to know which physical factors such as light, temperature, currents, oxygen, have influenced the presence or absence of a species of Syllids. The cases of inquilinism and monophagy lead us to suppose that the distribution of many species of Syllids depends upon the presence of some particular species, which are utilized for food, and upon particular types of nutrition. During three years of research it has been possible to observe that each species has a certain time of reproduction. Some species do not experience interruptions, carrying on reproduction through the year, while in others reproduction takes place only for one or two months of each year. It has not been possible to compare the periods of sexual maturity in the Mediterranean and in the Atlantic because the data of the Authors on the reproduction period of Syllids in the Atlantic are scarse. We know only that Syllis prolifera and Trypanosyllis zebra are ripe in the Gulf of Naples in winter while in the Atlantic they are ripe in summer and spring. A concise study of little morphological characters pointed out that Syllids have a very large intraspecific variability; the examples of ecological and geographical subspecies are in fact very frequent. These data permit us to suppose that some species considered hitherto to have a very large distribution or to be cosmopolitan, must not be considered as monotypic but as polytypic, perhaps sometimes constituting an Artenkreis. As has been observed in other animal groups, there are indications of the presence of different races due to reproductive biology. Although it is not possible to breed Syllids in the laboratory, these observations are suffìcient to show that the great adaptivity which permit these Polychaeta to adapt themselves to different environments may be due to extreme plasticity of their genotypic constitution.