1896.] NATURAL SCIENCES OF PHILADELPHIA. 409 



surface and the edge of the mantle. The process is not absolutely 

 continuous, but is carried on at more or less frequent intervals when 

 the animal is in a state of rest. At times when deposition is going 

 on, the margin of the mantle is in a more extended state than usual, 

 reaching to a point where the extremely thin and delicate mem- 

 brane is in contact with the extreniest margin of the already formed 

 shell. The glandular epithelium of the edge of the mantle secretes 

 less lime than that of the surface behind it, and is chiefly responsi- 

 ble for the periostracum of the shell, while the rest of the mantle 

 has the task of secreting the more limy matter which makes up the 

 bulk of the calcified shell. As the margin expands or contracts 

 over the still viscous secretion, the ornamentation of the mantle 

 edge, cilia, papillse, fringes, etc., everything which by its form or 

 bulk varies the flatness of the filmy membrane itself, mechanically 

 influences the form of the surface over which it passes, as the teeth 

 of a rake leave shallow furrows over the gravel of a garden walk. 

 Essentially in this way are the spiral stride, the revolving threads 

 and similar ornamentation developed on the surface of a fresh water 

 gastropod. The transverse sculpture, usually known as incremental 

 lines, arises from the periodicity of secretion, while ribbing or spin- 

 ose ornamentation originates in a periodic turgidity of the mantle 

 (how induced normally is not known) which rhythmically affects 

 that organ, and by its tidal rise and subsidence causes the shell 

 secreted during such epochs to be more inflated or capacious than 

 at the corresponding intervals. These features and modes of growth 

 can be observed in an aquarium with the more common fresh water 

 gastropods. 



It is a matter of common observation that alkaline salts, dust and 

 dryness are very inimical to land and fresh water mollusks. Salts of 

 chlorine and lime or soda will destroy slugs or snails subjected to 

 their influence ; the creature exudes a copious protective mucus up 

 to a point when exhaustion results and death soon follows. The tis- 

 sues under the action of such agents contract violently, shrivel, and 

 finally die. Against hot pure dry air and dust the slug protects 

 himself by burrowing and secreting a protective coccoon of limy 

 mucus, which dries to a leathery substance preventing further evap- 

 oration. The shell-bearing snail retreats into its house and closes 

 the door with a succession of almost air-tight epiphragms of which 

 the outer one, is often applied to a stone, a bit of bark, or the sur- 

 face of a tree or shrub, either on the branches or leaves. The com- 



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