RANDOM NOTES ON NATURAL HISTORY. 



The Shell - Bearing Mollusca of Rhode 

 Island. 



[BY HORACE F. CARPENTER.] 



Chapter III. 



Class 2. Pteropoda. 



The pteropods are all small mollusks, 

 some of them microscopic ; they are com- 

 monl3' known as sea-butterflies and whale 

 food. They spend their entire existence 

 in the open ocean, but are sometimes 

 seen at rare intervals drifted upon the 

 shore. They are brought up by the 

 dredge, in the fine sediment from the bot- 

 tom of the sea, at great depths. In the 

 arctic regions, they exist in immense num- 

 bers, discoloring the waters for miles. They 

 form the principal food of the Greenland 

 whale and of many sea-fowl. These ani- 

 mals are carnivorous ; they have heads, 

 ])ut no eyes ; some species are destitute of 

 a shell, others are protected by an external, 

 testaceous, or membranaceous covering of 

 variable form, with or without opercula ; 

 still others have an internal shell. They 

 ascend and descend with great rapidity, 

 and swim by means of a pair of wings 

 situated near the mouth. The different 

 species swim near the surface at certain 

 definite hours of the day and night, re- 

 maining but a short time, then sinking, 

 to be seen no more until the following 

 day at the same hour. As one species 

 disappears, another takes its place, and 

 they pursue this endless routine through- 

 out the whole twenty-four hours. One 

 rare species is only seen at 12 o'clock, 

 p. M. 



The sexes are united in the same in- 

 dividual. The}^ comprise two orders, The- 

 cosomata and Gymnosomata ; five families, 

 Hyaleidae,CymbuliidjB, Limacinidae, Cliidse, 

 and Eurybiidoe. There are forty-seven 

 genera and sub-genera, and more than two 

 liundred species, recent and fossil. 



The pteropods are infrequent visitors to 

 the New England coasts ; /Sjyirialis flevi- 

 ingii occurred at Nahant, Mass., in great 

 numbers during the summer of 1863. Mr. 

 Alexander Agassiz gives an interesting ac- 

 count of their habits in the Proc. Bost. Soc. 

 Nat. Hist., vol. x., p. 14. 



Class 3. Gasteropoda. 



These animals are the highest in the scale 

 of being of all the Mollusca, excepting the 

 Cephalopods. They occur in immense num- 

 bers, and are represented everywhere ; in 

 the ocean, in all fresh water lakes, ponds, 

 rivers, and stagnant ditches ; in swamps, 

 in lowlands, and upon the highest .moun- 

 tains ; in forest, meadow, and in gardens ; 

 on trees, on walls, and sometimes, even in 

 our houses. The species are numbered by 

 thousands, almost every island in the ocean 

 having species peculiar to itself. They are 

 also found on the bottom of the sea, on the 

 sands of the shore, attached to rocks and 

 sea-weeds, and floating in raid-ocean ; one 

 species, Janthina fragilis, secretes a shell, 

 white on the upper half, and of a beautiful 

 violet color on the lower portion. The 

 animal constructs a raft or float, several 

 times the length of its shell, and loads it 

 with eggs. The animal has not the power 

 of sinking and rising to the surface, but 

 floats along with its raft of eggs attached to 

 its foot. These shells are tiirown upon the 

 shores of Europe and America by storms, 

 and as they are liable to be driven upon our 

 coasts (they have been found on Nantucket) 

 I include them among Rhode Island shells. 



The Gasteropods have well developed 

 heads, with eyes and tentacles. The lower 

 part of the bod}^ is thickened into an ex- 

 panded, creeping dise, called the foot, by 

 which they crawl along, dragging the shell 

 after them, to which they are attached by 

 the mantle. They cannot leave, or come 

 wholly out of their shells, and as the}' grow, 

 they enlarge the shell by secreting carbonate 

 of lime from the food on which they live,, 

 and adding layer after layer of this material, 

 as their increasing size demands more room. 



I will explain here the meaning and ap- 

 plication of some of the terms to be used in 

 the descriptions of the Gasteropods in these 

 papers, by using for an example a common 

 fluviatite shell, Vivipara contecta. When 

 first hatched from the eggs, these animals 

 are provided with a shell about one-fourth 

 of an inch in length, which grows quite 

 rapidly b}- the addition of calcareous mat- 

 ter, as described above. The point where 

 the shell commences to grow is called the 

 apex; this, in some species, is decollated, 

 or cut off when the animal arrives at ma- 

 turity. The animal retires from the extreme 



