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The name of this common waterflea, Peracantha means "with spines on" (cantha - thorn,spine, acantha - spined) and truncata is, of course, "truncated". The little crustacean indeed seems a bit truncated when compared to the most, round shaped species of water fleas, and there is a row of spines on each "truncated" end of the scale halves. The scale also has a nice ribbed surface. What the purpose of these facts is, I don't know. The frontal margins of the scale has spines which are curved outwards. Maybe they lend better support when the waterflea is clamping on to the stems of waterplants. According to some investigations it is in any case a adaption to their way of life¹). The slit of the scale is wide, so the waterflea may sit firm on the waterplants and providing a large "scraping surface".
Peracantha is a genus within the Chydoridae family, a group of waterfleas living on waterplants and on the bottom. The family is named after the roundly shaped Chydorus species. The large and very common Eurycercus lamellatus is also a member of this family. Peracantha scratches on waterplants to scrape of algae and other small food particles, for that purpose the legs are provided with claws and bristles. This waterflea is also able to swim with its second pair of antennae, but when crawling they are not useful and folded away under the scale, making the waterflea less conspicuous to its predators. The head has two eyes above each other: a large compound eye and a smaller, single eye, called a nauplius eye. (A Nauplius is a larval stage of crustaceans having such an eye).