Ptekopoda- 



-MOLLTJSCA.- 



-Pteropoda. 



357 



fins for progressive motion. They are found far at sea, 

 ill large herds, swimming about in a lively manner by 

 undulatory or flapping movementsof their membranous 

 fins, wliich open and close like those of the butterfly 

 while basking in the sun. They shun the lightsome 

 day, and sink, as the sun rises, into the bottom of the 

 deep, to attain that shaded gloom which suits them; 

 but on the evening's approach they gradually again 

 ascend to the surface, and regain all their vivacity ; 

 so that if some will fancifully seek amongst them the 

 niolluscan analogues of insect-butterflies in their manner 

 of natation, they will not fail to mark the correspond- 

 ency between them and the moths in their crepuscular 

 and nocturnal liabits. The little Hyales {Cavolimdai) 

 first appear about five in the afternoon. When the 

 garish eye of day begins to grow dim, two or three 

 species venture upwards to the field of their occupancy; 

 as evening advances several small species of Cleodores 

 (Ckodora) rise in great number with other Hyales and 

 Atlantes, but the larger kinds do not leave the abyss 

 and mingle in the crowd until night lends them her 

 friendly veil ; and some species, as the Hyalwa balan- 

 tium (Dtihintium recitrvuin) are even so fearful of the 

 light's malign influence, that they do not come to the 



surface excepting when the night is very dark. After 

 a few hours' disport the lesser species begin to descend 

 and disappear ; the larger follow at a little later hour, 

 so that towards midnight only a few wandering indi- 

 viduals can be taken. These may possibly remain even 

 to the dawn, but the sun's rise is the signal which reoals 

 them to their home. After this not a single Pteropod 

 is to be seen, either at the surface or at any depth to 

 which the eye can penetrate. Each species has its 

 own time at which it rises up and goeth down, deter- 

 mined not by the clock, as you will readily believe, but 

 by the degree of obscurity in the heavens, so that in an 

 overshadowed day they rise earlier than in a cloudless 

 one, and sink earlier also to repose." 



The Pteropods are carnivorous animals, feeding on 

 minute cnistaceans, as the Entomostraca, &c., and 

 small Medusae. They form great part of the food of 

 the whale, in high latitudes, and are also devoured in 

 great numbers by many sea-birds. 



The Pteropoda are divided into two groups or orders 

 according as they are furnished or not with a shell. 

 Those in which the body is inclosed within a thin 

 shell, are called Thecosomata \ and those whicli are 

 destitute of any such appendage, Gymnosomata, 



Okder I.— thecosomata (Shell-be.\ring Pteropods). 



The species of Pteropoda belonging to this order are 

 distinguished by having their body inclosed within a 

 shell. 



In most of the species the body and shell are straight 

 or globular, and there is no operculum ; but in one 

 fimily {Limacitiidai), the animal and shell are spiral, 

 and there is a spiral operculum present. 



Family— CAVOLINIDiE (= HyaMdce). 



The Ily.alajas have a calcareous, sj-mmetrical shell, 

 which is sometimes straight, at others curved ; some- 

 times globular, and at others needle-shaped. The 

 animals have two united fins, without any posterior foot- 

 like appendage between them. It is by means of these 

 fins that the animals swim through the water; and it 

 has been observed that when they are touched, they 

 retract them and fall to the bottom of the vessel in 

 which they were detained as captives. 



The family consists of five distinct genera : — 



1. IIyal.ea (= Cavolina), with a globular trans- 

 lucent shell. Represented on Plate 11, fig. 28 [Ihjidcca 

 tridentala). 



2. DiACRlA, with an elongated triangular shell. 



3. Cleodoua (=Clio), with an elongate, pyra- 

 midal, hyaline, three-sided shell. 



4. Palantium, with a triangular, depressed, trans- 

 versely-waved shell; and 



5. Stvliola (= Ci£i-.si;i.s), with a slender, elongate, 

 conical shell of a subcylindrical form. 



Fa-mily— TRIPTERIDiE (Cuvieriidw). 

 The Ciivieras have a glassy, transparent, cylindrical 

 shell, with a transversely ovate, simple aperture, and 



(in the young) an acute apex, which, however, falls off 

 in the adult, and is separated from the anterior cavity 

 by a partition. The animal has an elongate, cylindrical j 

 body, and has two large lateral wings or fins in front, 

 united below by two small lobes. The genus Cuvieria 

 (= Triptera) is the only one in the family, and contains 

 three recent species and one fossil. 



Family— CYMBULIID^. 



The Slipper Shells have a cartilaginous sort of shell, 

 shaped very like a slipper, pointed in form, and trun- 

 cated posteriorly. The animal is globular or ovate, 

 and has large, rounded fins connected ventrally by a 

 small intermediate lobe. This family contains four 

 ])eculiar-looking, pelagic genera, of which the genus 

 Cymhidin is the type. The shell which incloses tho 

 animal is of a gelatino-cartilaginous nature, of an oblong 

 figure, and shaped very like a slipper. It contains four 

 species. " During the day," says Mr. Adams, " these 

 animals must occasionally descend a considerable depth, 

 one having been brought up attached to a thermometer 

 of a sounding line, during the voyage of H.M. ship 

 Samarancf, from one hundred and fifty fathoms in tho 

 South Atlantic." 



Family— LIMACINID^. 



The Liraaciuas have a small transparent, sinistral, 

 spiral shell, with the mouth angularly produced on tho 

 columella side, and closed by a distinct, spiral, vitreous 

 operculum. The animal has a spiral, sinistral bod)', 

 with fins attached to the sides of tho mouth and united 

 ventrally by a lobe which bears the operculum. 



