In the _Cephalopoda_, from κεφαλὴ,, _head_, and ποῦϛ, _foot_, the locomotive apparatus consists of arms, or tentacles, which surround the mouth in numbers more or less considerable. The Cuttle-fish (_Sepia_), and the Poulpes (_Octopoda_) are types of this last class. The Molluscous Gasteropoda have the organs of respiration formed for aerial respiration, or for respiration under water. This physiological arrangement involves important differences in internal organization in these molluscs, and renders it convenient to divide them into two secondary groups; namely, _Pulmonary Gasteropods_, which breathe in the air, and by a species of lung, and _Non-pulmonary Gasteropods_, which breathe in the water, by means of branchiæ or gills. CHAPTER XIV. PULMONARY GASTEROPODS. The _Pulmonary Gasteropods_ comprehend those molluscs which, as we have said, live in the air and breathe the natural atmosphere. The respiratory organ is a cavity in the walls of which the blood-vessels form a complicated network. The air enters this cavity through an orifice, which the animal opens and shuts at will--a species of lung, in short, which is placed upon the back of the animal. They are both terrestrial and aquatic animals. In the latter case, they must come to the surface of the water in order to breathe, like the phocas and cetacea among the Mammifera. The Pulmonifera, the second order of Gasteropods, comprehends those animals which live in and breathe the air. It is divided into four sections; the Operculata, or animals whose shells are closed by an operculum, and the In-Operculata, or animals without operculum. Operculata is divided into two families; first, Aciculidæ, types, Geomelania and Acicula; and second, Cyclostomidæ, types, Pupina and Cyclostoma. Cyclostoma is perhaps the best known; the mouth is circular, the name being derived from _cyclos_, circle, and _stoma_, mouth. The second section, In-Opercula, contains five families. First, Auriculidæ, types, Conovulus and Auricula. Second, Limnæidæ, types, Planorbis, Physa, and Limnæa. Third, Oncidiadæ, types, Vaginulus and Oncidium. Fourth, Limacidæ, types, Testacella and Limax. Fifth, Helicidæ, types, Clausilia, Pupa, Achatina, Bulimus, Succinea, Vitrina, and Helix. LIMNÆIDÆ. The Limnæidæ, _Aquatic Pulmonary Gasteropods_, is the second family of the series. They belong to the group that come to the surface of the water to breathe, as do the cetacea and phocas among the Mammifera. The _Limnæa_, _Planorbis_, and _Physa_ are the principal members of this group.