340 LOWER INVERTEBRATES. 



it has a long proboscis and two contractile feelers, with the eyes at their bases. The 

 lingual teeth are seven in a transverse band. 



The Cyclostomes are largely tropical, but very few species straying into temperate 

 regions. They live in damp places, some on the ground, some in trees, while others 

 are found far from the sea. Over a thousand species have been described. Some 

 have a peculiar gait ; the foot is divided into halves by a longitudinal furrow, and in 

 walking the animal advances one side and then puts it down, then the other side is 

 moved forward in the same way ; the two sides corresponding to the two feet of man. 

 The principal genera are Cyclostonia, Cyclophorus, Cyclotus, and Chondroponia. 



In the AcicuLiD^ the shell is nearly cylindrical, the margins of the aperture being 

 nearly parallel. From the wave-like motion with which they progress, they have been 

 termed ' looping-snails.' The species are amphibious, and live among the sea-weeds 

 thrown up on the shore, or in shallow water. The species are small. The only genus 

 which needs notice at our hands is Truncatella, in which, as the animal approaches 

 maturity, the upper parts of the spire are broken away and the animal repairs the 

 damage by closing up the broken whorls by a calcareous deposit. On account of this 

 truncation of the shell, the genus has received its name. The family contains about a 

 hundred species, mostly from the tropics. 



The Paludinid^ shares with the Limnseidfe, already mentioned, the common 

 name, ' pond-snails.' Its numbers are widely distributed through the temperate zone 

 of the northern hemisphere, but few being found within the tropics. They live in 

 muddy ponds or streams, where they crawl slowly over the bottom 

 or even burrow in the soft mud. The foot is large and broad, a 

 well-developed proboscis is present, and the long cylindrical ten- 

 tacles bear the eyes on little projections near the base. The water 

 required for respiratory purposes is conveyed to the gills by an 

 interesting contrivance. On either side of the neck are developed 

 little fleshy outgrowths which, together with the mantle, form 

 ier<eito, showing res- little tubes. In the adjacent figure these are seen on either side, 

 pira ry u es. projecting a little outside the shell. The water goes in through 



the right tube, passes over the gills, and then is forced out through the tube on the 

 left. 



The sexes are distinct, and the young are brought forth alive. The eggs undergo 

 their development inside of the mother, and in the species with thin shells they may 

 occasionally be seen inside the body. The development requires about two months, 

 and at the end of that period the young are sent forth, three or four at a time, to 

 begin life for themselves. The shells are thin in some forms and more solid in others ; 

 the prevailing color is some shade of green or greenish brown, banded with darker, 

 though sometimes the bands lacking. 



The principal genus, Paludina, has been divided up into several sub-genera, of 

 which Melantlw and Tulotoma are strictly North American. A curious instance of 

 the fact that a low temperature, which affects so greatly the land shells, has but little in- 

 fluence on the size of fresh-water forms, is found in the case of Paludina ussuriensis, 

 the largest Siberian species, which occurs in a latitude but little south of the line of 

 perpetual frost, and where the mean annual temperature is the same as in Iceland. 

 On the other hand, recent explorers have brought home large species found in the 

 lakes of Central Africa. No species have as yet been found in Australia, New Zea- 

 land, Polynesia, or the West Indies. 



