1. Accurate measurements were made of adults, larvae and eggs of Rhabditis pellio, representing an average selection for comparison with Johnson's figures.

2. The most satisfactory method of culturing the nematodes was in hanging drops, in a medium free from an excess of putrefying bacteria.

3. A medium poor in food was found to prolong life (up to 40 days), while a rich medium quickened up the life processes thereby causing death sooner. It is probably partial starvation which prevents the larvae from becoming mature while in the living earthworm.

4. In a medium favourable for prolonging life, the males were found to live about one-third as long as the females (F generation). It is probable that the females only live for about 4–8 days in a decaying earthworm after attaining maturity.

5. The maximum number of eggs laid per female was 257 (F generation) and 197 (F1 generation). It is probable that between 150 and 300 eggs are laid per female in the natural state. No information was obtained regarding the effect of the various media on egg production.

6. In most cases the females died when spent of eggs.

7. Vivipary is considered to be due to a favourable environment probably aided by a high temperature.

8. Larvae were found to be more susceptible to cultural conditions than the adults, especially those (F generation) which were not bred from females in culture media. The growth-rate of F1 generation larvae was found to be slightly faster in a natural medium, such as earthworm broth, than in peptone.

9. The chief food is considered to be bacteria.

10. During this work R. pellio behaved entirely as a bisexual species. A few cases of copulation were observed, showing that some of the males had not lost their “sexual instinct.”

11. Rhabditis pellio is considered to be a species in which hermaphroditism is just commencing and in which true males and females exist together with hermaphrodite females whose numbers fluctuate, the degree of hermaphroditism possibly bearing some relation to the sex ratio.

12. The sex ratio was about 1 ♂: 2 ♀♀, in both the F and F1 generations, but varied considerably. There were a few cases in which the male proportion was equal, or higher, than the female.

13. Four different species of British earthworms were examined, each being infected by Rhabditis pellio to a varying extent. Eisenia foetida was, however, rarely infected. Size of the earthworm, in proportion, played no part regarding the degree of infection. There is an indication that the head (segment I to the beginning of the clytellum) is the most heavily infected region.

14. A few specimens (larval stage) of an Ascarid, a species of Porrocaecum, a parasite of small mammals and birds, were found in three of the species of earthworms examined.

15. A probable course of the life history is given, based on this work and that of Johnson (1913).