The material on which the following notes are founded was collected during a short residence at the Marine Biological Laboratory, Millport, in August, 1908. A comprehensive survey of the marine fauna was impossible in so limited a time, but a fair idea of the parasites of some of the common fishes was obtained. I have to thank the Government Grant Committee of the Royal Society for enabling me to undertake this work. It is also necessary to gratefully acknowledge the courtesy of several members of the West of Scotland Marine Biological Association and of Mr Elmhirst, the director of the Millport Laboratory.

From the results of our experiments we may conclude that the development of T. lewisi outside the invertebrate host is not confined to one species or genus, but may take place at least in the rat louse and the rat flea and also (though perhaps more incompletely) in the bed-bug. We see further that development needs not always to be combined with longevity in the invertebrate host, but that sometimes life without development may be longer than with it (behaviour in the tick compared to that in the louse).

When we compare the behaviour of T. lewisi in the four invertebrate hosts studied here, we see that the most complete cycle takes place in the flea, where forms are produced which are never found in cultures. In the louse the development may be truly described as a natural culture; in the bug, the development (as far as we could judge by our incomplete experiment) does not even produce all the cultural forms (only the crithidiae) and at last in the tick the trypanosomes do not develop at all but are only preserved for some time.

We finish this paper by thanking Professor Nuttall for the liberal way in which he enabled us to carry out these experiments, and for the keen interest he always took in our work.