Biology of Finfish and Shellfish (2+1)

4.10.2. Trochophore

The ciliated trochophore in its typical state appears in Palella and Trochus and in these it is formed at a very early period, even before the formation of the mesoderm, and becomes free at once. It is pear shaped and measures about 0.5 mm in length. A circle of preoral cilia, the prototroch or velum divides the body into two unequal parts, the upper one consist of prostomium whereas the lower part bearing mouth and anus. The preoral part is large and convex with an apical plate bearing the long cilia called the apical cilia as seen in the larvae of patella, dentalium etc.

Near the apical cilia, there are two ciliated elevations each consisting of a single cell. At the posterior end, there are large anal cells bearing a bunch of cilia called telotroch. Between the ciliary girdle and anus, on the dorsal side, the area of ectoderm constitutes the shell glands which secretes a thin, saucer shaped larval shell. The trochophores of lamellidae consist of an additional shell inside the first one which will be cast off later. A slight ridge in the neighboring area of shell represents the border of mantle. Alimentary canal is made up of single layered epithelial cells in the form of a tube. It comprises mouth, stomodaeum, oesophagus, stomach and intestine (mesenteron). A single large cell gives rise to the mesoderm. A rudiment of radula is present in oesophagus. The trochophore contains two cerebral ganglia, a pair of eyes from the underside of apical plate. The statiolith sacs appear as depression of ectoderm at the sides of the mouth. Soon the trochophore develops into a yet more efficient locomotor form the veliger.