An Introduction to the Invertebrates, Second Edition - tiera.ru

174 CRUSTACEAFig.13.4 The diversity of Crustacea (continued). (a) Argulus foliaceous (Branchiura), ectoparasitic on fish: some150 species, less than30 mm in length; (b) Cephalobaena tetrapoda (Pentostoma) from a snake lung. Ninety species of these very much modifiedcrustaceans are known, living as parasites in the nostrils or lungs of reptiles or other vertebrates.They were formerly placed ina separate phylum. (c) Cyclops sp. (Copepoda): there are some 8500 species and vast numbers of individuals of copepods in themarine plankton (and a few in fresh water), forming an important basis for many food chains. (d) An ostracod, about1mm in length,enclosed in a bivalve carapace.They are rowed along by their antennae, in the sea or in fresh water.1000 species of woodlice). Habitats allowing semi-terrestrialexistence were probably crucial to the transition: present-day semiterrestrialcrustaceans include the fiddler crabs (e.g. Uca), whichspend much of their time in the air but retreat to water-filledburrows, and isopods such as Ligia, seeking out damp crevices highon rocky shores.Terrestrial crustaceans have body fluids with high osmoticpressure, which at once suggests that they evolved not by way offresh water but overland from the sea, perhaps over salt marsh orthrough mangrove swamps.Land crabs are relatively large and impermeable invertebrates,not immediately liable to dry out. The stiffened gills do notentirely collapse in air and therefore allow some gaseousexchange, and part of the very vascular gill chamber functionsas a lung. However, land crabs must return to the sea to breed,unlike amphipods and isopods, which retain their eggs ina brood pouch where they develop directly.