228 NATURAL HISTORY. 



or two of their shells are met with in collections ; one especially, Carinaria, a beautiful little glassy 

 boat, which one would take at first for some form of Paper Nautilus. The shell of Carinaria gives 

 110 idea, however, of the form of the animal which, with one or two allied genera such as Pterotrachea 

 and Firoloides, which do not produce shells at all is sometimes abundant in calm weather on the 

 surface of the warm seas. The shell hangs below the animal, connected with it by a kind of neck, and 

 is merely meant for the protection of some very vital organs, including the heart, the gills, and the 

 liver. The remainder of the animal is ten times the size of the shell, and forms a large sac, usually 

 gelatinous and very transparent, often dotted over with purple pigment spots. The front of 

 the sac is drawn out into a Ijng, singularly-formed snout, and near it there are bright, well-marked 

 eyes, and a pair of feelers. The posterior part of the sac is produced into a fin-like tail. Along the 

 upper middle line of the animal, in the position in which it swims in the water, the part corresponding 

 with the ' foot ' in ordinary shell-fish is raised into a high crest-like fin. The bodies of these creatures 

 are large, some of them not less than five or six inches in length, but, like most free-floating animals, 

 they are very soft, formed mainly of ' connective tissue,' with little in it but sea-water. In this way 

 their bulk is greatly increased without materially adding to their weight, and they weigh little more 

 than an equal bulk of sea-water, and require little exertion to float or swim. 



" One curious result of this transparency is that we can see through the outer wall, in the most 

 wonderful detail, all the internal arrangements the nervous centres, with the complicated organs of 

 sense, the heart, with its pulsating chambers, and the blood following its coarse through the system 

 and through the gills, the alimentary canal, and all its accessory glands. The Heteropoda are 

 probably the most highly-organised group in which such transparency exists. 



" The shells of Carinaria are rare in the Globigerina Ooze ; but two small spiral shells, belonging 

 to animals of the Atlanta peronii and Oxyyyrus keraudrenii, are sometimes in such numbers as to 

 have a sensible effect in adding to the formation. Although Heteropod shells of the present 

 day are insignificant in size, they played a much more important role in early times, for there seeins 

 little doubt that the great shells of the genera Euoinphalus and Bellerophon, which sometimes go 

 far to make lip whole beds of limestone of the Silurian and Carboniferous periods, are to be referred 

 to this group." * 



The Carinarice are found in the warmer parts of the Atlantic and Indian Oceans. They feed 

 on Acalephse and Pteropods. Five species are described. 



Genus Cardiapoda has a minute cartilaginous shell ; animal like Carinaria ; habitat, the 

 Atlantic Ocean. Five species are known. 



FAMILY XLVI. ATLANTID./E. 



In the genus Atlanta, the shell is small and glassy, with a prominent keel; the aperture is 

 narrow and deeply notched. Fifteen species occur in the warmer parts of the Atlantic. 



CLASS III. PTEROPODA. 



The Pteropoda, or "Wing-shells," are a small group of animals, whose entire life is passed in the 

 open sea, far away from any shelter, save what is afforded by the floating Gulf-weed, and whose 

 organisation is specially adapted to that sphere of existence. In appearance and habits they 

 strikingly resemble the fry of the ordinary Sea Snails, swimming like them by the vigorous flappings 

 of a pair of fins. To the naturalist ashore they are almost unknown, but the voyager on the great 

 ocean meets with them where there is little else to arrest his attention, and marvels at their delicate 

 forms and almost incredible numbers. They swarm in the tropics, and are no less abundant in Arctic 

 seas, where by their myriads the water is discoloured for leagues. (Scoresby.) They are seen 

 swimming on the surface in the heat of the day, as well as in the cool of the evening. Some of the 

 larger kinds have prehensile tentacles, and their mouths are armed with lingual teeth, so that, fragile 

 as they are, they probably feed on still smaller and feebler creatures (e.g. , Entomostraca}. In. high 

 latitudes they are the principal food of the whale and of many sea-birds. Their shells are drifted on 

 shore, and they abound in the fine sediment brought up by the dredge from great depths. 



* Sir "\Vyville Thomson's " Voyage of the Challenger," Vol. I., p. 121. 



