140 BATHYMETRICAL SURVEY OF 



Diaptomus, Daphnia, and Bosmina are occasionally found, but the 

 species included in the preceding list are so much more common, that 

 when the generic name only is mentioned it will be understood that 

 the common species is referred to. 



This small association of animals and plants constitutes what may 

 be called the lacustrine type of plankton. A not very dissimilar 

 association is found in small ponds, but the species for the most part 

 are different. The Diaptomus may be D. castor, the Daphnia D. pulex, 

 the Bosmina B. cornuta; Rotifers and Algae will be more abundant and 

 varied, and there will probably be some Ostracodes. It might have 

 been expected that the shallowest lochs would have had a plankton of 

 the pond type, but it has been found that even the smallest lochs 

 surveyed had the plankton distinctly lacustrine. A few nearly or quite 

 stagnant lochans showed a slight approach to the pond type in the 

 presence of Bosmina cornuta and Volvox and in the abundance of 

 Rotifers and Algae. 



The remarkable variations of the loch trout, which have so much 

 puzzled naturalists cannot be touched on here, but parallel cases are 

 found among the smaller animals. Diaptomus gracilis varies remark- 

 ably in colour, and is usually constant for each loch, and several other 

 Entomostraca vary greatly in size and form; chief among these is 

 Daphnia. The typical lacustrine form of this genus, which will be 

 referred to as Daphnia lacustris, has an evenly rounded head with a 

 depression on the line of the forehead marking off the brow from the 

 beak. Where this depression is obliterated the head of the animal has 

 a very different appearance, resembling that of a parrot. The form 

 differing most from the typical Daphnia lacustris is that in which 

 the head is produced upwards into a sort of peak or helmet. For 

 convenience, this form will be referred to as Daphnia galeata, though 

 it is doubtful if the points of difference are of specific value, and 

 intermediate varieties are found. 



After Daphnia, the species which varies most is Bosmina 

 obtusirostris. The typical lake-form has a short mucro at the posterior 

 angle of the valves. It varies much in size and in colour, being usually 

 hyaline, but sometimes purple, or rarely orange and purple. 



During its season Holopedium, from its large size, is very conspicuous 

 in those lochs in which it occurs. It is frequently so abundant that it 

 chokes up the nets in a short time, and makes it impossible to get a fair 

 proportion of the other animals present. It appears in some lochs as 

 early as May, and continues till August. 



Commonly a single organism, usually vegetable, will so increase in 

 a loch as to form what the Germans call a " Wasserblut." The Algae 

 Clathrocystis, Oscillaria, Botryococcus, Anabcena, and Volvox are among 

 those which most frequently increase to this extent, but almost any of 

 the smaller organisms, as Diatoms, Rotifers, or Protozoa, may on 



