The _Atlantidæ_, _Pterotracheidæ_, and _Carinariidæ_, at various times recognized as forming a distinct sub-class or order of Gastropoda, under the name of Heteropoda or Nucleobranchiata, are now regarded as families of aberrant Gastropods organised for swimming in the open sea. The _Atlantas_ are found in great numbers in warm latitudes, and are provided with a glassy, thin, flat, spiral shell, not unlike a keeled Ammonite. The glassy shell of the _Carinaria_ is one of the most beautiful structures of any mollusc, and at one time was such a rarity that £100 are said to have been given for a single specimen, which at the present time is perhaps worth only from five to ten pounds. Species of _Carinaria_ are found in the Mediterranean and warmer parts of the Atlantic and Indian Oceans. The animal is large, semitransparent, and elongate, with a compressed fin-like foot which projects from the body, and is used in swimming. The gills are placed towards the hinder part of the back and covered by the shell. They feed on jelly-fish of various kinds, and probably on other soft animals. [Illustration: Fig. 15. Glassy Nautilus (_Carinaria lamarcki_). _a_, proboscis; _b_, tentacles; _c_, shell; _d_, gills; _e_, foot; _f_, sucker. ] Section EUTHYNEURA. [Sidenote: Cases 94–135.] The Gastropods belonging to this sub-class have the visceral nerve-loop straight and not twisted as in the STREPTONEURA. All the EUTHYNEURA are hermaphrodite, and their radula is generally composed of numerous similar denticles on each side of a median tooth. Scarcely any of these forms are provided with an operculum in the adult state. The EUTHYNEURA may be divided into two orders, _Opisthobranchia_ and _Pulmonata_. Order 1.—OPISTHOBRANCHIA. [Sidenote: Cases 94–97.] All the Molluscs of this order are marine, some (_Tectibranchia_) breathing by means of the ordinary Gastropod ctenidium, which is generally behind the heart, whereas others (_Nudibranchia_) have developed a different type of respiratory organs. The _Opisthobranchia_ include the “Pteropods” formerly considered as a distinct class, the “Bubble-shells” (_Bullidæ_), the “Sea-Hares” (_Aplysiidæ_), the “Umbrella-shells” (_Umbraculidæ_), the _Nudibranchs_ and some others. [Illustration: Fig. 16. Shell-bearing Pteropod (_Cavolina tridentata_). Case 96. _a._ Shell and animal. _b._ Side view of shell. _c._ Dorsal view of shell. ] The Pteropods[4] are sometimes called Sea-butterflies, and are organized for swimming freely in the ocean. They have a pair of fins developed from the sides of the mouth or neck, which perform a flapping movement during progression. Some Pteropods (Thecosomata) are provided with small glassy shells; others (Gymnosomota) are naked. They exist in countless millions in some parts of the ocean, discolouring the water for miles. They constitute the principal food of the Baleen Whales. [Illustration: Fig. 17.