Case 94.
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.
Fig. 15.
Glassy Nautilus (Carinaria lamarcki).
a, proboscis; b, tentacles; c, shell; d, gills; e, foot; f, sucker.
Section Euthyneura.
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.
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.