Chiton integument: development of sensory organs in juvenile Mopalia muscosa.

The girdle epidermis of adult Mopalia muscosa secretes several types of structures, including calcareous spicules and innervated hairs. Newly metamorphosed chitons superficially resemble adult animals, but they lack the adult girdle ornaments, shell sculpture, and coloration. The morphogenesis of the adult girdle structures has not been described previously for any species. Juvenile Mopalia muscosa secrete hairs at metamorphosis, but it was not known if these hairs were sensory or if they were retained as the animals grew. I discovered that the hairs of juveniles become the tips of adult hairs. When juvenile hairs are detectable by light microscopy the sensory components already exist, suggesting that they are functional receptor organs. The other girdle ornaments of young juveniles, the primary calcareous spicules, are lost as the animal grows. I also demonstrated that the hairs are not uniquely innervated; the same sensory structures are produced in conjunction with other girdle ornaments on the marginal and ventral faces.