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

SEA, FRESH WATER AND LAND892. Active reabsorption of ions in the excretory organ, withthe production of urine more dilute than the blood (hypotonic), mayreinforce ion uptake at the surface. This occurs in many but byno means all freshwater animals.Reproductive adaptationsSperm do not survive well in fresh water and external fertilisation is rare.More freshwater animals are hermaphrodites. The eggs typically contain muchyolk and most develop directly: a fragile larva, small with a relatively largesurface area, would be very vulnerable to osmosis/salt loss and liable to besweptdownstream.Freshwater wascolonisedonlyinpartby the aquaticroute up estuaries.Manyfreshwater animals (nearly all of the molluscs and arthropods) evolved fromterrestrial forms secondarily returning to water.LandMany so-called terrestrial invertebrates live in soil (e.g. earthworms, planarians,nemertines, nematodes) orothermoistplaces, andremainphysiologicallyaquatic.Problems for animals colonisinglandinclude:1. Avoidance of desiccation2. Tolerance or avoidance oftemperature fluctuations3. Support and locomotion of the body in air.Not surprisingly, mosttruly terrestrialinvertebrates are arthropods or molluscs.When surrounded by dry air rather than water animals must:1. Respire usinglungs rather than gills (see Box 9.2)2. Excrete with minimal water loss (see Box 9.1)3. Perceive different environmental stimuli, often with different senseorgans.RoutesbywhichlandwascolonisedMany small animals (about 2 mm long) came by the interstitial route, evolvingfrom ancestors living in the spaces between grains of sand or other sediments.Most terrestrial animals, however, came to land either by the aquatic routefrom fresh water (e.g. vertebrates) or by the overland route from the top ofthe seashore (e.g. most terrestrial arthropods and many molluscs). The routesmay be identified by differences between present-day animals:1. Animals once adapted to fresh water have body fluids more dilutethan the sea and this difference will be maintained in their furtherevolution (e.g. the body fluids of slugs and snails range from 97 to231mosm).2. Terrestrial animals whose ancestors took the overland route arelikely to have body fluids nearer to marine concentrations (e.g.700 mosm in the woodlouse Porcellio). Animals evolving by way ofthe top of the seashore are also likely to tolerate some evaporativewater loss.