Ontogenetic plate and plate field development in two chitons,Middendorffia andIschnochiton

The larva ofIschnochiton gradually develops 6 transversal ridges and later a seventh one on its back. These intersegmental ridges are composed of two types of cells, one of them a goblet cell, as they are found in the girdle. Both types of cell seem to be responsible for cuticle secretion. The depressions between the ridges, called plate fields, are responsible for plate secretion. There are two different cell types. The marginal ones, which remain slender and tall, develop an enormous flat villus (here called a stragulum). Each plate field is tightly sealed against sea water by the stragulum. In the crystallization chamber formed by the stragulum, the first plate anlage is formed. In the later stages, the stragulum protects the growing plate margin. No periostracum is found. Most probably the cells developing the stragulum are responsible for the secretion of the tegmentum and in early development are the only secretory cells. The central cells of the plate field, which very soon flatten, then produce the second plate layer, the hypostracum. There is no secretion product in common to plate fields and intersegmental ridges. During the developmental period studied inIschnochiton (from hatching to 94 h) the number of cells of the individual plate field is not increased.The earliest anlage of the plates inMiddendorffia andIschnochiton is rodlike with straight margins and is not composed of several granules. Growth takes place at least inMiddendorffia by apposition of lime to the ends, posterior and, preferably, anterior margins. All plates are posttrochal.