35fi 



PlIYSA.- 



-MOLLUSCA.- 



— PTEKOrODA. 



Genus Physa. — The Bubble Snails liavo a sinistral 

 shell, of an ovate form, spiral, thin, and polished. The 

 aperture is oval, rounded in front, the inner lip spread 

 over the last whirl, and the outer lip acute. The species 

 nia}' easily be recognized by their bright polished sur- 

 face, and their sinistral or left- handed whirls. The 

 I'hysa; are chiefly found in clear running streams and 

 in fountains, seldom in stagnant water. They appear 

 to be very quick in their movements, and when touched 

 or alarmed, they only arrest their progress for a moment, 

 and then go on again without further hesitation. When 

 floating on the surface of the water, they possess the 

 power of letting themselves down gradually by means 

 of a glutinous thread affixed to the surface (as in the case 

 o'i Lit inpa — see p. 204) and when kept in a glass vessel, 

 they have been observed frequently to poise themse'ves 

 on their foot, and rapidly vibrate their shells. The 

 Physa; are not so numerous in species as the Lymncecf, 

 but are widely ditfused, being found in Europe, North 

 America, South Africa, India, and the Philippine 

 Islands. This family and genus are represented on 

 Plate 7, fig. 1 [Plnjsa rivalis). 



The Genus Planorbis differs very much in ap- 

 pearance from the two preceding genera. The shell 

 is dificoidal, instead of spiral, with a depressed spire 

 and numerous whirls, which are visible on both sides. 

 The mouth is crescentic, and the peristome is thin, 

 sharp, incomplete, the upper margin projecting. The 

 species are numerous, upwards of sixty having been 

 described, and are in a great majority of instances 

 natives of temperate climates, as Europe and North 

 America, &c., though a number are found also in India 

 and China. Locomotion in these animals is slow; and 

 it has been observed in the case of a common British 

 species [P. cm-neus) that when irritated, it discharges 

 a purple fluid from under the margins of the mantles, 

 similar to that of the Scalaria (see p. 200), but which is 

 of so fugitive a nature that no means tried yet have 

 been able to preserve it. 



TnALASsoPHiLA — SIarine Snails. — In this group 

 the animals have no distinct tentacles, or they are 



expanded so as to form a frontal disc, upon the front 

 part of which are placed the eyes. They are either 

 marine in their habits, or they live in the vicinity of 

 the sea. 



Family— SIPHONAPJID.'E. 



The Siphonarias have their frontal disc Lilobed an- 

 teriorly, and the respiratory orifice covered by a large 

 fleshy lobe of the mantle. The shell is conical, and 

 exacfly resembles a patella, except that the horse-shoe- 

 shaped muscular impression is divided on the right side 

 by a deep siphonal groove, which extends in the form 

 of a projection somewhat beyond the margin. In form 

 it is depressly conical, with the apex subcentral, and 

 recurved posteriorly. There is only one genus, Sipho- 

 naria. The species, upwards of thirty in number, are 

 found adhering to rocks, like limpets, between tide 

 marks, and are widely extended in their geographical 

 distribution. They are most numerous within the 

 tropics, being fuund in considerable abundance in India, 

 the Philippine Islands, South America, the Cape of 

 Good Hope, Australia, New Zealand, and the islands of 

 the Pacific. The family is represented on Plate 7, 

 figs. 1, 2, 3 [Siphonaria radiata and (jrandis). 



Family— AMPHIBOLID.E. 



Tlie Amphibolas have a subglobose, spiral, umbili- 

 cated shell, with an oval mouth and a rather expanded 

 outer lip, which is sinuated behind. The surface of the 

 shell is singularly uneven, and battered as it were. 

 The animals possess an operculum, which is horny and 

 subspiral. 



Only one genus, Ampiiibola, exists. 



The Amphibolas are found living in salt marshes near 

 the sea. They exist in great abundance in New Zea- 

 land and Australia, living in pools of brackish water, 

 and at particular seasons burying themselves in the 

 sandy mud. The natives of New Zealand use them as 

 articles of food. 



Class III.— PTEROPODA (the Pteropods). 



The true position of the Ptei'opoda in the natural 

 arrangement of molluscous animals, is still somewhat 

 undefined. Some conchologists place them between 

 the Brachiopods and Ccphalopods, others between the 

 Gasteropods and Brachiopods. Lamarck conceived 

 they should be placed between the Gasteropods and 

 Bivalves, and M. Blainville is of opinion that they are 

 in reality only a tribe of Gasteropods, allied to the 

 BuUidce. M. Souleyet supports this view of their posi- 

 tion, though the structure of the animal evidently 

 entitles them to rank as a distinct group or class. The 

 animals resemble very much the larval or young state 

 of the Sea-snails ; and the shell, when present, is dis- 

 tinctly univalve and analogous to the shell of the Gas- 

 teropods. They are, however, much inferior to this 

 class of mollusks, and we place them therefore after 

 the Univalves in our arrangement. 



The animal in most of the species is inclosed in a 

 thin shell, which is either conical, cylindrical, or sub- 

 globular. It is symmetrical, and consists of a dorsal 

 and a ventral plate united, with an anterior, transverse, 

 contracted opening for the head, and lateral slits for the 

 passage of certain long filiform processes of the mantle. 

 The sexes are united in the same individual. 



The Pteropods are amongst the most beautiful of all 

 the Mollusca, but are seldom seen alive except by those 

 " whose business is in the great deep." In consequence 

 of the great fragility of their shells, they avoid the 

 shore, and are generally met with only in the open 

 ocean. They are the most entirely natatory of all the 

 animals of this kingdom. " Created," says Dr. John- 

 ston, " to occupy the high seas, they are organized in 

 evident aptitude to the place assigned them, with a 

 light shell, which affects not their buoyancy, and with 



