98 



NATURAL HISTORY. 



Its colors are 



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Fig. 90. — Naked mollusc (Dendronotus arborescens). 



as it crawls over the Laminaria and the plant-like hydroids. 

 brown and white, in varying shades. 



These naked molluscs are so called because they have no shells in the 



adult ; but it is per- 

 fectly evident that 

 they have descended 

 from shelled forms, 

 for in the young 

 they have a deli- 

 cate shell, almost 

 exactly like that of 

 forms which are 

 shelled through life. 

 Some of their near- 

 est relatives are 

 always shelled, but 

 none of them are 

 of especial interest. 

 We figure the shell 

 of one of our east- 

 coast species as the 

 type of a genus (Bulla) embracing the bubble-shells, which 

 are very common in collections. 



An enormous number of species of gasteropods, unlike 

 the forms just mentioned, are adapted for breathing air. 

 Inside the mantle cavity is a chamber, in the walls of which 

 tie- blood-vessels ramify, ami which, both in structure and ^eu^Sa?! 6 * 

 fund inn. closely simulates the lung of higher animals; and 

 this tad rendi rs the name Pulmonates especially applicable to them. The 

 •at majority of them are terrestrial in their habits, and most of the 

 remainder arc inhabitants of fresh water, only a very few being found in 

 the vicinity of salt water. 



The name snail, though used rather indiscriminately, in reality belongs 

 to the species of the Linnean genus Helix, of which some thirty-five hun- 

 dred forms have been described. Snails occur everywhere, 

 from the poles to the equator, but reach their highest devel- 

 opment in the tropics. They may be found in dense woods 

 or in the open fields ; in greenhouses and in cellars ; and 

 even given to climbing trees. They are almost exclusively 

 tarians. although occasionally a taste for flesh crops out, and once 

 in a while a snail will show that he is not above eating; his fellows. The 



Fig.92. Garden- 



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