Weismann[33] has pointed out that the sexual forms, with their property of producing eggs which can endure desiccation, recur most frequently in species such as _Moina_, which inhabit small pools liable to be dried up at frequent intervals, while the 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 determination 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_ reproduces more slowly when exposed to green light, and that individuals 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. =Summary of Characters of the British Genera.=[34] Tribe I. CALYPTOMERA, Sars.—The post-cephalic portion of the body enveloped in a free fold or carapace. A. Six pairs of thoracic feet, the first pair not prehensile (CTENOPODA).