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 Daphniidae 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 hght 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. 
