THE LIFE CYCLE OF TRICHINELLA SPIRALIS: II. THE MUSCLE PHASE OF DEVELOPMENT AND ITS POSSIBLE EVOLUTION

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Abstract
Larvae from gravid female Trichinella spiralis in the intestines of experimentally infected mice migrated through the peritoneal cavity and connective tissues to reach the muscles about the 6th day after infection. Larvae began to coil in the muscles about the 17th day, became resistant to pepsin digestion on the 19th, and began to form cysts on the 21st day. Larvae did not molt during the extraintestinal phase. Because T. spiralis molts only twice in the intestine and not at all in the muscles, two hypotheses are postulated to explain their mode of development. The first postulates that originally two molts occurred in the extraintestinal phase and that during the evolution of the parasite the necessity of these two molts gradually disappeared so that the infective larva may be considered to be a modified third-stage one. The second hypothesis postulates that in the absence of molting in the extraintestinal phase, the infective larva is in the first stage. It is further suggested that because only two molts occur during the life cycle of T. spiralis, the so-called adults are actually third-stage neotenic larvae.