[41] mSTEUCTIONS FOR COLLECTING MOLLUSKS — DALL. 



outer surface of it being touclied with a drop of mucilage. The oper- 

 culum can then be laid upon this in its natural position and the muci- 

 lage and cotton will retain it so without making it difficult to remove 

 for an examination of the shell, if desired at any time. For the preser- 

 vation of eggs of mollusks when they have a horny or calcareous shell, 

 small glass tubes securely corked are the best receptacles. Most of 

 these eggs are so small that they may be preserved in a dry state or in 

 alcohol without trouble, but the eggs of some of the tropical land snails 

 are so large that it will be necessary to drill a small hole and extract 

 the fluid contents as if they were bird's eggs in order to preserve them. 

 Such eggs are best preserved in alcohol. 



MARINE SHELLS. 



The preparation of marine shells for the cabinet does not essentially 

 differ from that required for land or fresh- water shells, except that in 

 the marine shells the muscular system is often much more strongly 

 developed and the creatures themselves much larger than the fresh- 

 water forms, and the manipulation is therefore somewhat more difficult. 

 The marine forms are also more apt to be incrusted with foreign bodies, 

 bored by predatory sponges, like Oliona, or even by other mollusks, or 

 perforated by certain annelids which have the power to dissolve the 

 lime of which the shell is composed, and in this way secure a retreat, 

 for themselves. 



Shells which do not contain the living animal are frequently occu- 

 pied by hermit crabs or by tubicolous annelids. The latter fill up the 

 larger part of the spire with consolidated sand or mud, in the center 

 of which they have their burrow. The hermit crabs do not add any- 

 thing to the shells which they occupy, but, on the contrary, by their con- 

 stant motion are apt to wear away the axis or pillar of the shell, so- 

 that often a specimen of this sort may be very fairly preserved and yet 

 on the pillar show characters entirely different from those which one 

 would discover in a specimen which had never been occupied by a crab. 

 A shell which the crab has selected for its home is often taken posses- 

 sion of, as far as the outside is concerned, by a hydractinia, a sort of 

 polype, which produces a horny or chitinous covering which is very 

 difficult to remove from the shell to which it is attached. As the hydrac- 

 tinia grows it finally covers the whole shell, to some extent assumes its 

 form, and then, if the creature has not attained its full growth, this is 

 apt to take place around the edges of the aperture, which are continued 

 by a sort of leathery prolongation which assumes in a rough way the 

 form of a shell. The crab, when he grows too large for the shell in 

 which he has ensconced himself, is usually obliged to abandon it and 

 find a larger one, which is always a difficult and more or less danger- 

 ous operation ; but if his shell is overgrown by the polype referred to, it 

 often happens that the polype and the crab grow at about an equal rate, 

 so that the latter finds himself protected and does not have to make a 



