A study of two distinct forms of Chaetogaster limnaei (Baer) was carried out in a population of Lymnaea pereger. One form inhabits the kidney of freshwater snails and the other lives on the external surface of these molluscs. Various differences between the two forms were studied.
The outer form possesses a significantly greater number of setae per bundle than the kidney form and those of the kidney form are also shorter than those of the outer form. The stomach and intestinal epithelia are considerably thicker in the outer form than in the kidney form.
The contents of the stomach and intestine of both forms was recorded from samples taken over a period of 12 months. It was seen that whilst the outer form fed on various planktonic organisms, the kidney form fed exclusively on cells derived from the kidney. Kidney forms would not under any of the experimental conditions feed on the planktonic organisms normally taken by the outer form.
Both forms were found to be specific in their habitat in that if removed from their host and introduced to another, kidney forms always colonized the kidney and outer forms always remained on the outer surface. Neither lack of planktonic food nor conditions of drought could induce the outer forms to enter the kidney.
Both forms showed a positive reaction to the mucus of their host but that of the outer form was by far the most pronounced. It was shown that the mucous trail left by the host snail assisted the outer form in finding a new host in that if the right direction was taken initially it would follow this trail, overhaul the snail and attach itself to it. The outer form also had greater ability to survive free living than the kidney form. Outer forms were kept alive for a maximum of seven weeks and kidney forms for only three weeks.
It was suggested that the kidney form be considered a subspecies of Chaetogaster limnaei and that it be known as Chaetogaster limnaei vaghini. A brief description of the new subspecies is given.