MOLLUSCS. 339 



Another member of the family which should be mentioned is Lacuna vincta. It 

 has a thinner and more slender shell than any of our Littorinas, and is reddish or horn- 

 colored usually, with two or more darker reddish bands, which follow 

 the spiral of the shell. Like the rest of the family, it is a vegetarian, 

 feeding on algae. 



Closely allied to the Littorinidse is the Rissoid^, which needs but a 

 passing mention. The family, and its principal genus, Rissoa, derive their ^^^'J^ll^^ui^^' 

 name from Risso, a naturalist who in the early part of this century studied 

 the fauna of the Mediterranean. Some of the members inhabit the sea, while others live 

 in brackish water, or even in that which is entirely fresh. In all, the spire is long, the 

 lip thickened, and the aperture rounded. Rissoella, which is found in Europe and 

 Japan, lives between tide-marks. The shell is very thin, and the eyes, which are placed 

 upon the surface of the head, are " so far behind the tentacles that the transparency of 

 the shell seems to be essential to the vision of the animal." The species of Missoa are 

 numerous, several being found in American waters. JBythinia is one of the most 

 prominent of the fresh-water genera. Its fifty species belong to the eastern hemisphere. 

 In the United States, Amnicola is distributed through all parts of the country, the 

 small species living in fresh water. They were formerly included in the Paludinidae. 

 Pomatiopsis should be mentioned, from the fact that the species are air-breathers. ' 



The Ctclostomid^ are exclusively terrestrial, and were formerly included arnong 

 the Pulmonata. Like the members of that order, they breathe the air by an essen- 

 tially similar pulmonary organ, but in the rest of their anatomy they deserve a place 

 where we have put them. This is one of the many instances where a too close atten- 

 tion to one organ or one physiological operation would lead to erroneous ideas of rela- 

 tionship. Still the other process is next to impossible, and on this point we can do no 

 better than quote the words of Fritz Mtlller : " Of a hundred who feel themselves 

 compelled to give their systematic confession of faith as the introduction to a manual 

 or monographic memoir, ninety-nine will commence by saying that a natural system 

 cannot be founded on a single character, but has to take into account all characters, 

 and the general structure of the animal, but that we must not sum up these characters 

 as equivalent magnitudes, that we must not count, but weigh them, and determine the 

 importance to be ascribed to each of them according to its physiological significance. 

 This is probably followed by a little jingle of words in general terms on the compara- 

 tive importance of animal and vegetative organs, circulation, respiration, and the like. 

 But when we come to the work itself, to the discrimination and arrange- 

 ment of the species, genera, families, etc., in all probability not one of 

 the ninety-and-nine will pay the least attention to these fine rules or 

 undertake the hopeless attempt to carry them out in detail." Not- 

 '^^'t'o^Tnca'^"'' withstanding this melancholy picture, science is constantly striving to 

 arrange the groups of animals and plants ; facts of structure are 

 weighed, and it is by just this process that the Cyclostomid89, a family without gills 

 and with a pulmonary respiration, are accorded a position here among the branchiate 

 molluscs. 



In this family the whorls of the spiral shell are rounded and the aperture is circu- 

 lar, hence the family name. As in the branchiate forms with which they are associ- 

 ated, the members of the family close the shell by an operculum which is round and 

 increases in size with the growth of the animal, the new additions being placed in a 

 spiral manner, the nucleus being central. The animal is much like that of Littorina ; 



