March 12, 1897.] 



SCIENCE. 



425 



ciliated infusoria ; artliropods ; (copepods, 

 amphipods ; ostracods ; decapod larvse and 

 larval traclieata) ; rotifers ; annelid larvse ; 

 ctenophores ; medusae. In its general char- 

 acter it more closely resembles Haliplank- 

 ton than Limnoplankton, the marked ex- 

 ceptions being the presence of rotifers and 

 the absence of cladocera. 



It has been frequently observed and re- 

 corded that copepods come to the surface in 

 vast numbers at night. We have frequently 

 observed that on certain days they are at 

 the surface in equal abundance. Their 

 presence at the surface appears to be inde- 

 pendent of light and darkness, or of 

 meteorological conditions, but correlated 

 with the presence at the surface of certain 

 species of diatoms, or of quantities of algal 

 d^bi'is; observations confirming the belief 

 that these diatoms and amorphous organic 

 materials are the principal food of copepods 

 and of young decapod larvse. 



Rotifers occur in great abundance dur- 

 ing July, August and September, but we 

 have found them at the surface only during 

 the day, and near the bottom during the 

 night. 



Cordylophora and a nudibranch mollusc 

 are found in water whose specific gravity 

 never rises above 1.005. 



Investigations are now in progress to dis- 

 cover the cause of the phenomena noted by 

 us, that ctenophores and medusae (Dadylo- 

 metra) , which are brought into the pond by 

 the tide, are checked in their growth, and 

 after several months of residence in the 

 pond show but a very slight increase in 

 size. The same causes have possibly re- 

 sulted in the various species of Nereis 

 Balanus, and molluscs described as inhabit- 

 ing only brackish water, and which differ 

 from similar marine species mainly in their 

 smaller size. 



Our earlier methods of plankton collection 

 were by means of fine nets, and by sand 

 filtration of known volumes of water after 



the method of Henson, Reighard, Sedgwick- 

 Eafter, Peck and others, but these have 

 been superseded by use of the Planhtonokrit, 

 invented and described by Dr. C. S. Dolley.* 

 The centrifugal method is a distinct ad- 

 vance, and materially reduces the error 

 when dealing with all organisms thus far 

 met with, except the Cyanophycese. But 

 with steam power it is confidently expected 

 that enough centrifugal force can be de- 

 veloped to throw out even these. 



The machine is particularly valuable as 

 a rapid, sure method for collecting the 

 microscopic plankton, and its use will dis- 

 close many forms hitherto rare or unknown. 

 As used by us, the two reservoirs, each of 

 one litre capacity, are filled with water 

 drawn from a known depth by means of a 

 valved tin tube. For control purposes both 

 reservoirs are used. After revolving 2 to 5 

 minutes the volume of organic matter is 

 read on the graduated tube ; the tubes are 

 then unscrewed, and the contents washed 

 out by a pipette and filtered distilled water 

 into a tube of narrow lumen graduated to 

 j-^^ of a cc. After settling for the neces- 

 sary time, either with or without treatment 

 with Formalin, the volume is read and 

 compared with the volume noted upon the 

 graduated tube of the reservoir. This is 

 necessary from the fact that certain forms 

 are packed more closely than are others by 

 the centrifugal force. The volume of water 

 is then made 5 cc; the organisms are dis- 

 tributed evenly by gentle shaking or by a 

 pipette, and the number of individuals of 

 each species is enumerated according to the 

 Sedgwick-Eafter method. 



Nocturnal Protective Coloration of Mammals, 

 Birds, Fishes and Insects. A. E. Veerill. 

 Much has been written in respect to the 

 imitative and and protective colors of mam- 

 mals, birds, insects, etc., and the bearing 

 of these facts on natural selection, to which 

 *Proo. Acad, of Nat. Sci. Philadelphia, May, 1896. 



