The nephridia, in fact, on this view, are ectodermic ingrowths, the coelomoducts coelomic outgrowths.
In the marine Streptoneura they are ectodermic projections which ultimately fall off; in the Opisthobranchs they are closed pouches; in Paludina and Bithynia they are canals as in Pulmonata.
The inner layer of the polypide-bud gives rise to the structures usually regarded as ectodermic and endodermic, the outer layer to the mesodermic organs.
After the formation of the gastrula by epibole the larva becomes enclosed by an ectodermic test covering the whole of the original surface of the body, including the shell-gland, and leaving only a small opening at the posterior end in which the stomodaeum and proctodaeum are formed.
The shell-gland is bounded by a ridge of ectodermic cells.