29 



Reproduction among the Entomostraca, more especially among 

 the Cladocera and Ostracoda, is very commonly parthenogenetic. 

 In some species, and even genera, this is the only known method of 

 reproduction. Usually, however, asexual reproduction does not go 

 on indefinitely, because at certain seasons at least the males make 

 their appearance. At the same time as the appearance of the males, 

 the females among the Cladocera produce special eggs, known as 

 winter or resting eggs, which require to be fertilised before they will 

 develop. These eggs are also very frequently provided with pro- 

 tective cases formed from the shell of the mother, and known as 

 " ephippia." By means of these peculiar structures the eggs are 

 enabled to withstand, if need be, the cold of an Arctic winter, or the 

 heat of an Australian summer. This accounts for the fact that from 

 the sun-baked mud of dried-up ponds Professor G. O. Sars, working 

 in Christiania, has' been able to make a most magnificent series of 

 additions to the known Australian Entomostracan fauna. 



To the question " Where are Entomostraca to be found ? " the 

 general answer is "Wherever there is water." But of course all the 

 different kinds of Entomostraca do not occur everywhere. There 

 are, as usual, special forms in special places. There are some 

 species, for example, which thrive only in the middle waters of lakes. 

 These are the so-called " plankton " forms. They are all more or 

 less transparent, and even glass-like. They are mostly provided with 

 powerful swimming organs, and very often, also, with long spines and 

 such like outgrowths, which serve probably to increase their power of 

 remaining suspended in the water without undue effort. Other species 

 go to the opposite extreme. Instead of always swimming, like the 

 plankton forms, they never, or but rarely, swim at all. They live in 

 or on the mud at the bottom of lakes, ponds, etc., and, as might be 

 expected, they exhibit curious modifications of structure adapting 

 them to this particular mode of life. Others again, and these form, 

 perhaps, the majority of the Entomostraca, while good swimmers for 

 short distances, usually prefer to cling to weeds and other objects in 

 the water. Some few of this class have also found out how to take 

 advantage of the peculiar physical properties of the surface-film of 

 water, and they may be seen at times hanging from the under side of 

 the surface like flies on a ceiling. 



[Sketches and diagrams illustrating some of the above-mentioned 

 species were exhibited and briefly explained.] 



The role of the Entomostraca in nature may be best discovered 

 by observing on the one hand what they feed upon, and on the other 

 what aquatic animals (and plants) feed upon them. The Entomos- 

 traca live chiefly on minute Protozoa and Algae, together with small 

 organic particles derived from the decomposition of larger animals 

 and plants. Seeing that they occur in enormous numbers, there can 

 be little doubt that they play a very important part in keeping our 

 ponds and ditches in good condition. In their turn they are preyed 

 upon by a whole series of animals, ranging from the Protozoa to the 



