ORDER TECTIBRANCHIATA.
(Covered-gilled Mollusks.)
In general form and appearance, many of the animals contained in this Order resemble those of the preceding; but they differ from them in having the gills attached along the right side, or upon the back. These organs assume the form of a single plume, or of leaflets more or less divided, but not symmetrical; they are always more or less covered by the mantle, whence the name applied by Cuvier to the Order. The mantle encloses within its substance, in almost all cases, a small shell to protect the vital parts. In some of the genera, the shell is developed to such a degree as to cover the animal; as in the fresh-water limpet (Ancylus), found in some of our streams. For the most part, however, the species are marine; they are widely scattered, but appear to be most numerous in the Indian and Mediterranean Seas. The sexual functions are united in each individual, in which particular, this Order agrees with those which I have already considered; but in the form of the breathing organs, it manifests a closer affinity with the following Order.
The Covered-gilled Mollusca may be grouped in five families, three of which are represented by British species.
Family Pleurobranchidæ.
The members of this family a cursory observer would at once associate with the Nudibranchs; but
