GOVERED-GILLED SUBGROUP 3335 



Gastropoda, assigned them a position between that class and the bivalved Molluska. 

 Dr. Morch and others, long ago pointed out the affinity of the Pteropods with the 

 Opisthobranchs, and Dr. Pelseneer has couie to the conclusion that these mollusks 

 should be included in two of the Tectibranchiate groups of the Opisthobranchs. 

 The Pteropods seldom come near laud, except when driven by currents and tem- 

 pests, and although they rise to the surface principally at night, they have been ob- 

 served during the daytime. They are characterized by two delicate fins or lateral 

 expansions of the foot {parapodia), placed at the anterior end of the animal, and 

 used in swimming, being moved with considerable rapidity, like the wings of a 

 butterfly. When they wish to sink, their fins are contracted, and the anterior part 

 of the body, in some cases, is more or less withdrawn into the shell or the mantle, 

 and they thus fall to the depth desired. At times they remained stationary, by 

 keeping the fins merely extended. Some Clio, Pneumoderma, for example ad- 

 here to floating bodies by means of oral appendages provided with suckers, others by 

 means of a sufficiently developed foot. The fins, termed epipodia by Huxley, are 

 sometimes united, forming a single disc, sometimes partly connected, or, finally, 

 entirely separated. The Pteropods are hermaphrodite, or have the sexes united in each 

 individual. They deposit their ova in the form of long bands which float on the 

 surface of the sea. They are carnivorous in their habits, and live upon minute 

 animals which abound in the sea, including small Molluska, and Entomostraca and 

 other Crustacea. Some of them possess prehensile organs for seizing their prey, 

 but many appear to be totally unprovided with any special appendages for this pur- 

 pose. In the early stages of development a shell always exists, but when adult, 

 only certain forms are furnished with such protective covering, the others being 

 naked mollusks. They possess a heart, consisting of auricle and ventricle, inclosed 

 within a pericardium. The branchiae are either internal or external. The head, 

 distinct in some groups, and furnished with one or two pairs of tentacles, is practi- 

 cally wanting in others, and reduced to a mere mouth at the base of the fins. Nearly 

 all the various forms have a radula, and some horny jaws. There are no eyes, or 

 these are only represented by minute pigment dots upon the visceral sac, or on the 

 tentacles. The fry of the Pteropods closely resembles that of ordinary Gastropods; 

 but subsequently the frontal veil of the Pteropod disappears, and is replaced by the 

 parapodia or permanent fins. 



The number of species of this group is inconsiderable, and may be estimated at 

 about fifty-four; but, as regards individuals, the numbers are inconceivable. The 

 bottom of the sea in various parts of the Gulf of Mexico, the Bay of Biscay, and the 

 Mediterranean, is paved with an accumulation of the dead shells of Cavolinia > Cleo- 

 dora, and other forms. Their tiny forms occur in shoals in the Tropics, and in more 

 temperate seas, and, even in Arctic latitudes, they exist in such myriads as to dis- 

 color the water for considerable distances. They there form a large item in the diet 

 of the Greenland whale. Some of the species have a wide distribution, occurring 

 in the Atlantic, Pacific, and Indian Oceans, others are more localized. 



The Pteropoda are divided into two sections, namely, Gymnosomata 



and Thecosomata. The animals belonging to the former group have 



no mantle or shell in the adult state. The body is generally elongate, the head dis- 



