[37] INSTRUCTIONS FOR COLLECTING MOLLUSKS DALL. 



The mollusks obtained by use of tlie net are chiefly larval forms of 

 gastropods, which float on the surface until tbey reach a more advanced 

 stage of development — lanthina, pteropods, and the so-called hetero- 

 pods. True pelagic organisms are usually not found very near the shore, 

 unless some ocean current strikes in toward tlie shore in the vicinity, 

 but on short voyages, or long ones, large numbers of extremely in- 

 teresting and usually very rare species may be obtained in this way. 

 The length of time the net sbould be left over will depend upon the 

 weather and upon the region traversed. In the tropics a short time 

 will afford good results. In northern seas tlie net may sometimes tow 

 for hours without many specimens being obtained. The most favorable 

 time for its use is from a little before sunset to a short time after sun- 

 set, since many of these creatures seem to liave tlie habit of coming to 

 the surface at that time of the day and remaining at greater depths 

 during the rest of the twenty-four hours. At the same time it may be 

 noted that towing done at almost any time of day may produce good 

 results, for all of the species do not have the crepuscular habit. 



Many of these animals may be obtained alive, put into a tumbler of 

 sea water, and preserved in a living state for a day or two at a time if 

 the water is kept fresh. It is desirable that water-color sketches show- 

 ing the form and coloration of these pelagic organisms should be made 

 from life, as their tissues are very contractile and when jireserved in 

 alcohol shrink irregularly, so that the normal form is difficult to make 

 out. With these mollusks will be obtained many curious crustaceans, 

 and occasionally pelagic fish or fish larvse and pelagic animals like 

 Salpa and various medusse, most of which are never found near shore. 

 Several naturalists have experimented with towing nets which were 

 made to be sunk below the surface, closing by a peculiar mechanism, so 

 that at a given depth they could be opened in order to obtain animals 

 proper only to that particular depth. These experiments, however, 

 belong rather to the domain of deep-sea dredging and collecting, for 

 information in regard to which the inquirer is referred to the works 

 previously cited. 



PEESEEVATION AND PEEPAEATIO:^^ OF COLLEOTIONS. 



After the collector has brought home the siDoils of his excursioa there 

 is still a good deal to be done before the wet and dirty shells, covered 

 with parasitic growths or inhabited either by the original mollusk or 

 some hermit crab, will be ready to be placed in the cabinet. Some of 

 them, if living, may find a temi^orary place in an aquarium for the study 

 of their habits, but, for the most part, the collector will wish to prepare 

 his specimens either for anatomical use in the future or as dry speci- 

 mens for his cabinet. The preparation of mollusks for anatomical pur- 

 poses has been described in a special chapter of these instructions. 

 For ordinary rough work nothing is better than clean 90 per cent alco- 



