Crustaceans 



JELLY MOUNTS 



49 



nearest woman's stocking. What distin- 

 guishes tliese nets from all others is that 

 the lower end of the net, instead of being 

 tied off, is blocked by a small glass tube 

 tied firmly in place. These nets are towed 

 slowly through the water, a process which 

 results in the accumulation of large quan- 

 tities of plankton within the net. The net 

 itself is then very slowly and with constant 

 shaking lifted from the water with the 

 result that all those forms which have 

 been accumulated in the net are washed 

 down in the small glass tube, the contents 

 of which may then be tipped out into an- 

 other container. Unless one is on a very 

 long collecting trip, it is better to bring 

 home planktonic crustaceans alive, and 

 for this purpose the contents of the tube 

 should be tipped into at least a gallon of 

 well-aerated water — fresh or salt as the 

 case may be — for transference back to the 

 laboratory. It is almost impossible to make 

 a satisfactory preparation from the horri- 

 ble messes which result from endeavor- 

 ing to preserve directly the contents of an 

 entire tube by throwing it into formalde- 

 hyde, a technique too often employed. 

 When plankton samples are brought back 

 to the laboratory they may again be con- 

 centrated with a plankton net or other de- 

 vice, and the still-Uving individuals are 

 picked out with a pipet and transferred to 

 a small bowl of clean, well-aerated water. 

 Marine forms in particular should be 

 washed in numerous changes of water to 

 rid them of adherent phytoplankton which 

 is almost impossible to remove from the 

 fixed specimen. Though every mounter of 

 microscope slides will go to endless lengths 

 to narcotize many invertebrates, few ever 

 appear to consider this necessary in the 

 case of crustaceans. It is however much 

 easier to identify specimens if their ap- 

 pendages are properly spread out, and the 

 writer always kills crustaceans either ^vith 

 weak alcohol or with chloroform, a few 

 drops of which are sprinkled on the sur- 

 face of the water. As soon as they have 

 dropped to the bottom they will be found 

 to be flexible, and may then be picked out 

 and placed on a sUde, the appendages ar- 

 ranged more or less in the order required, 

 and 95% alcohol cautiously dropped on 

 them until they have stiffened in position. 



Such specimens may then he transferred 

 directly to a tube of do% alcohol where 

 they will retain the required shape until 

 needed for mounting. Some small crusta- 

 ceans are always found mixed up with 

 weeds, both marine and fresh-water, from 

 which they may be readily sei)arated in 

 the laboratory. Large masses of the weeds 

 are brought back to the laboratory, and 

 placed in shallow dishes with just sufficient 

 water to cover them. The lack of oxygen 

 in the water soon forces the crustaceans to 

 detach themselves and gather round the 

 edges of the bowl from which they may be 

 removed with a pipet. There are a few 

 marine copepods {Dyspontius, for ex- 

 ample) which are considered exceedingly 

 rare, for the reason that nobody ever col- 

 lects them. These forms have sucking 

 mouth-parts which they use to extract the 

 juices of algae. They never become de- 

 tached from these algae in the normal 

 course of events. They may be readily 

 collected by taking large masses of the 

 specific alga, placing it in weak alcohol, 

 shaking it vigorously, and then examining 

 the sludge which is deposited at the bot- 

 tom of the jar. Good hauls of these forms 

 may also often be found at marine stations 

 by going through the sludge which collects 

 at the bottom of the jars in which both 

 algae and ascidians have been stored. 



There are also many small crustaceans 

 which dwell in mosses, even in those which 

 are apparently quite dry. These will not be 

 secured if the moss is treated with a 

 Berlese funnel in the way described in 

 the last chapter. The only way to collect 

 them is to soak the moss in some kind of 

 narcotic until the crustaceans are stunned, 

 then to rinse off the moss in a considerable 

 volume of fluid which is allowed to settle, 

 and to examine the sludge. The same proc- 

 ess, apphed to marine sands between tide- 

 marks, often discloses small forms not 

 found by any other means. Another fruit- 

 ful way of collecting so-called rare forms 

 is to go netting at night, since there are 

 many marine crustaceans (Cumacea, for 

 example) which become planktonic only at 

 night. Parasitic forms, particularly cope- 

 pods, are also often overlooked, particu- 

 larly those which inhabit invertebrates. 



No matter how these forms are col- 



