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 alimentary tube consists of three regions: firstly, the anterior buccal mass with the oesophagus, of ectodermic origin, and therefore bearing cuticular structures, namely the jaws and radula; secondly, the mid-gut, of endodermic origin and including the stomach and liver; and, thirdly, the hind-gut or intestine.
The shell-gland is bounded by a ridge of ectodermic cells.
The shell-gland, as development proceeds, extends from its point of origin as an ectodermic thickening, which may be only slightly concave or may be deeply invaginated and then evaginated.
They agree with primitive nephridia in being of ectodermic origin, in consisting of perforated cells in linear series, and in having no communication with the coelom.
Spadella P. Langerhans, with a pair of lateral fins on the tail and a thickened ectodermic ridge running back on each side from the head to the anterior end of the fin.
The pre ponderating elements of the ectodermic layer are elongated columnar -?irii cells, each containing a nucleus, and bearing cilia at their free ex- -9°.
The immediate cavities or pits into which the tracheal stigmata open appear to be in many cases ectodermic in sinkings, but there seems to be no reason (based on embryological observation) for regarding the tracheae as an ingrowth of the ectoderm.
The cells forming the limbs of the ectodermic folds secrete nodules of calcite, and these, fusing together, give rise to six (or twelve) vertical radial plates or septa.