50 CRUSTACEA BRANCHIOPODA CHAP. 



species which produce sexual forms only once a year are all 

 inhabitants either of great lakes which are, never dry, or of the sea. 

 Many suggestions have been made as to the environmental 

 stimulus which induces the production of sexual individuals, but 

 nothing is definitely known upon the subject. 



We have said that even in those generations which contain 

 sexual males and females there are always some parthenogenetic 

 individuals ; there is therefore nothing in the behaviour of 

 Daphniidae, either under natural conditions or when observed in 

 aquaria, to suggest that there is any natural or necessary limit 

 to the number of generations which may be parthenogenetically 

 produced. 



The parthenogenetic Daphniidas are extremely sensitive to 

 changes in their surroundings ; small variations in the character 

 and amount of substances dissolved in the water are often 

 followed by changes in the length of the posterior spine, in the 

 shape and size of crests on the head, and in other characters 

 affecting the appearance of the creatures, so that the deter- 

 mination of species is often a matter of great difficulty. It is 

 remarkable that the green light which has passed through the 

 leaves of water-plants appears to have a prejudicial effect upon 

 some species. Warren has shown that Daphnia magna repro- 

 duces more slowly when exposed to green light, and that in- 

 dividuals grown in this way are more readily susceptible to 

 injury from the presence of small quantities of salt (sodium 

 chloride) in the water than individuals which have been exposed 

 to white light. 



The majority of the Cladocera belong to the floating fauna 

 of the fresh waters and seas ; a few are littoral in their habits, 

 clinging to water-weeds near the shore, a very few live near the 

 bottom at considerable depths, but the majority belong to that 

 floating fauna to which Haeckel gave the name of " plankton." 

 The Crustacea are an important element in the plankton, 

 whether in fresh waters or in the sea, the two great groups 

 which contribute most largely to it being the Cladocera and the 

 Copepoda. For this reason it will be more convenient to discuss 

 the habits and distribution of individual Cladocera and Copepoda 

 together in a chapter specially devoted to the characters of pelagic 

 faunas (cf. Chap. VII.). We will only add to the present chapter 

 a table of the families with a diagnosis of the British genera. 



