BULLETIN 39, UNITED STATES NATIONAL MUSEUM. [26] 



ENEMIES AND PARASITES. 



Sea mollusks are the prey of marine animals in general, and feed to 

 some extent upon each other. There are no special or peculiar enemies 

 except the boring sponge (Gliona), which riddles shells and other limy 

 bodies with a network of small canals and chains of perforations familiar 

 to every collector. Some Polyzoa erode the surface of shells to which 

 they are attached and when removed leave marks resembling the pits 

 on a thimble, often regularly arranged and sometimes taken for a normal 

 sculpture by the unwary or inexperienced student. 



COMMENSAL ORGANISMS. 



Various worms and Crustacea are commensal with mollusks, of which 

 the oyster crab {Pinnotheres) is a most conspicuous and familiar instance. 

 The shells infested by commensal worms are sometimes abnormally 

 modified by their presence. On the other hand, a curious little mollusk 

 (CocJiliolepis) has been described by Stimpson, which lives only under 

 the scales of an enormous annelid {Acoetes lupinus) common in the har- 

 T3or of Charleston, S. C. 



The collector whose tastes and opportunities lead him to study the 

 liabits of life of mollusks can hardly fail, if patient and accurate, to add 

 greatly to our knowledge. The literature of moUuscan biography, if it 

 may be termed so, is meager to an extraordinary degree. No better 

 afield for research can be imagined, and with experience it is found that 

 the (apparently) most trifling details may have an important bearing 

 and value. 



DEECaiNG. 

 THE CONSTRUCTION AND USE OF THE DREDGE. 



The dredge has been in use among naturalists since the last century. 

 The form commonly in use for many years was that invented by Otho 

 Frederick Miiller, a Danish naturalist, and it is this form which is de- 

 scribed in most of the text-books on conchology.* This instrument was 

 composed of a rectangular frame of iron, with two movable arms ex- 

 tending forward, and with two large pieces of rawhide attached by 

 wires to the hinder edges of the rectangle. The sides of these two 

 pieces of rawhide were caught together with wire or twine in such a 

 way as to make a net. This form of dredge is very inconvenient be- 

 cause, when not in service, the hide shrinks, dries, and becomes very 

 Tiard, requiring to be soaked a long time in order to be fit for use; it is 

 also difficult to turn such a net inside out in order to get at the con- 

 tents of the dredge. 



The modern dredge has been improved by Stimpson, and by the au- 

 thor of this paper. It consists, like the other, of a rectangular frame 

 (Fig. 1), the sides being shorter than the upper and lower portions of the 



* See Woodward's Manual, edition of 1871, p. 141, -figs. 33, 34. 



