BULLETIN 39, UNITED STATES NATIONAL MUSEUM, [42] 



change. It is supposed that the polype profits to some extent by the 

 microscopic animals attracted by the food or excrement of the crab, so 

 that this joint housekeeping is mutually beneficial, and, for such cases, 

 since the word parasite would not be strictly accurate, the word com- 

 7nensal has been adopted. These modified shells often assume very sin- 

 gular shapes. The polype is able in the course of time to entirely dis- 

 solve the original calcareous shell upon which its growth began, so that 

 if the spire be cut through it would be found throughout of a horny or 

 chitinous nature. Some of the older naturalists were deceived by 

 forms of this sort and applied names to them, supposing that they were 

 really molluscan shells of a very peculiar sort. 



In removing the animal matter from the shell of large gastropods it 

 will often require a good deal of time and care to get out all the animal 

 matter from the spire, but it is well worth while to take the trouble, as 

 the presence of such matter forms a constant attraction for museum 

 pests of all descriptions. A medium-sized syringe is convenient for 

 washing out the spire of such shells. The ordinary marine gastropods 

 maybe treated in a general way like the fresh- water gastropods. There 

 are, however, abnormal forms, especially among tropical species, which 

 require particular attention. Some species become affixed to corals and 

 •overgrown by them, retaining only a small aperture tlirough which the 

 sea water can reach the jirisoner. Such specimens are best exhibited 

 T:)y retaining a part of the coral and cutting the rest away, showiug at 

 once the mode of occurrence and the form of the covered shell. Bor- 

 ers are always more difficult to handle and prepare for the cabinet than 

 other mollusks. They are usually more or less modified for their ijeculiar 

 mode of life, and frequently rely upon their burrow as a protection, so that 

 the shell is reduced, relatively to the animal, to a very small size. Most 

 of these forms are best kept in alcohol. The hard parts may properly 

 be represented in the cabinet by other specimens. Some of the bivalves, 

 such as the American "soft clam," possess very long siphons, covered 

 i\^ith a horny epidermis, and it becomes a question as to whether an 

 attempt should be made to preserve this epidermis in the cabinet or 

 not. The writer has seen very nicely prepared specimens in which the 

 fleshy portions had all been taken out and replaced by cotton, so that 

 the epidermis of the siphon retained its original position and form; but 

 such specimens are always very delicate, easily broken, and liable to 

 attack by insects, so that it would seem hardly worth while to go to the 

 trouble, when si)ecimens may be preserved complete in alcohol showing 

 all the features referred to. Boring shellfish, like Pholas, frequently 

 have accessory pieces, which are liable to be lost when the soft parts 

 are removed unless care is taken to avoid it. Other bivalves have the 

 internal ligament reinforced by a shelly plate, which is called the ossi- 

 culum. This is very easily detached and lost, and, being an object of 

 great interest, special pains should be taken to x^reserve it, even if it 

 should become detached. 



