(Numbers given in the summary refer to certain key figures in the text which facilitate understanding.) An account is given of the functional morphology, ecology, habits and feeding mechanisms, of a representative series of anomopod cladocerans of the family Chydoridae, embracing some 22 species belonging to 15 genera. Of these genera two are defined as new on the basis of this investigation. Habits, which often depend on anatomical specializations of great complexity, are more diverse than has been supposed, and permit the exploitation of a diversity of ecological niches. The functional significance of many anatomical features is described. Carapace specializations, previously almost unstudied, are extremely important. Fundamental homologies not only within the Chydoridae but within the Anomopoda as a whole, can now be recognized and the feeding mechanism of all species can be related to one original type. By subtle modification of its various components, however, this basic mechanism has been adapted to very different ways of life and to different functions. The various species are described in a sequence which in part illustrates the progressive development of specialized ways of life rather than phylogeny, but light has been thrown on affinities, or lack of affinity, between certain species. Alonopsis elongata (3) exhibits primitive behaviour. It swims over, and frequently settles on, surfaces, especially of stones. When it settles it rests on the ventral carapace margins (4), which show little elaboration. Under favourable conditions it can crawl forward by means of the first trunk limbs alone, though when necessary the antennae are used as props to maintain balance and as levers to assist progression. They are also used for scrambling when required, and for this they are suitably armed. Food, whose nature is described, is collected from firm substrates by scrapers borne on trunk limbs 2 and 3. The trunk limbs and the complex feeding mechanism which they subserve are described (7). Entangling secretions are produced from trunk limbs 1 and 4. In balancing ability, and associated structural features of the carapace margins, Acroperus harpae and Camptocercus rectirostris exhibit the same grade of organization as Alonopsis, and have a similar feeding mechanism, but show marked divergence in the carapace-headshield complex. Their close relationship is, however, confirmed. These genera exhibit progressive specialization of the trunk/post-abdomen joint, Camptocercus being most specialized. Although no more adept at crawling than Alonopsis, Alona affinis has certain apparently trivial morphological features of great evolutionary potential, which have been exploited by more specialized crawlers. These and other anatomical features are described in relation to ecology, habits and the feeding mechanism. Comparison is made with three smaller species of Alona. By widening the embayment, development of a marginal flange ventrally, elaboration of the carapace setae (37), and associated specializations, Peracantha truncata (34 to 36) has become an extremely efficient crawler over surfaces. Its efficiency is such that assistance from the antennae is no longer required, though they still serve as efficient oars for swimming. As projecting antennae are vulnerable to small predators and impede efficient crawling, especially in confined spaces, they are folded and their extremities tucked within the carapace. Peracantha frequents the leaves of plants from which, by means of fine-toothed scrapers of the second trunk limbs, it removes finely particulate food. Appendages involved in feeding are shown in figure 47, and the feeding mechanism is described. This employs the same elements as Alonopsis, but differs in ways related to habits, habitat preferences and the nature of the food. Only labral gland secretions (48) are employed. Two species of Pleuroxus are very similar to Peracantha. Such specializations for crawling have been further exploited by Alonella exigua (50, 51) which has an even wider embayment, a broader ventral flange, and employs more marginal setae for balance. By virtue of its ability largely to seal the carapace chamber and vigorously to pump water from it, A. exigua is able to maintain a pressure difference between the water inside and outside the chamber which enables it to cling to and crawl over surfaces in any plane. Like a fly on a ceiling it can creep beneath leaves, such as floating leaves of Potomageton, and exploit a niche not available to its close relative A. excisa. It can even crawl inverted on smooth glass. Finely particulate food is collected by a feeding mechanism similar to that of Peracantha though different secretions are employed. Over short distances A. exigua is also a very efficient swimmer and attains, both relatively and absolutely, faster speeds than any chydorid yet tested. A. excisa cannot perform the feats of which A. exigua is capable. Anatomical bases of the differences are revealed. A. nana, the smallest anomopod, has different habits from those of A. exigua and A. excisa. It is tolerant of a wide range of conditions, is less specialized either for crawling or swimming, and cannot protect its antennae in the same way. Some aspects of small size are discussed. The animal known variously as Alonella rostrata and Rhynchotalona rostrata differs so markedly in its functional anatomy from the species of Alonella that it is assigned to a new genus, Disparalona. This species is specialized for scrambling among mud, usually in situations where a thin layer overlies a hard substratum, and to this end its antennae (56, 57) are specialized. Such a way of life also demands specializations of the carapace margins (58) very different from those of species which crawl over plants or stones. Most notable of its other anatomical features are specializations of trunk limb 3, whose gnathobase bears an enormously developed sweeping seta which in life is in almost...