54 Comparative Animal Physiology 



not extensively invaded salt water is their lack of mechanisms for secreting 

 salt outward. This is perhaps surprising in view of the ability of some of them 

 actively to absorb salt against a gradient. It must mean that inward and out- 

 ward permeability are physiologically separate. 



Ability to live in salinities greater than that of sea water Requires ability to 

 maintain a gradient of hypotonicity, as in brine shrimps. Some marine crabs 

 such as Uca are able to do this; a few others are hypotonic in normal sea water 

 where the ability can be of little value. Apparently all land crabs can keep 

 the blood hypotonic. Ability to maintain hypotonicity has developed several 

 times among arthropods. 



Little is known of osmotic relations in endoparasites, but some helminths 

 show a limited osmoregulation. Gregarines swell and shrink with changes in 

 the medium. 



Success in fresh water requires osmotic regulation of such order that a high 

 concentration of body fluids can be maintained against an extreme osmotic 

 gradient. Such regulation requires active mechanisms fdr (1) low permeabiHty 

 to water, (2) water elimination, (3) salt retention, and (4) salt replacement. 

 Fresh-water animals differ from marine in degree of development of these 

 regulating mechanisms. Copious water elimination is necessary in all fresh- 

 water animals except in very impermeable eggs. Marine protozoans put into 

 fresh water develop vacuoles or increase the output of vacuole systems already 

 present. Marine Crustacea which can live in brackish water increase their 

 urine output in dilute media. Fresh-water fish excrete a copious urine, where- 

 as in some marine fish the kidney becomes almost vestigial. But nowhere is an 

 animal known which can excrete a totally salt-free urine, hence there must 

 be mechanisms for salt retention and salt replacement. 



Salt retention can be brought about ( 1 ) by very low or negligible outward 

 permeability to body salts, and (2) by salt reabsorption in the excretory organs. 

 Low salt permeability is found in many marine animals. Salt reabsorption 

 occurs in those marine animals whose kidneys function in the conservation of 

 certain useful salts, but the power of salt reabsorption is much more highly 

 developed in fresh-water animals, for example in the long tubules of the kid- 

 neys of fresh-water crustaceans and vertebrates. 



Salt may be replaced from food or by active absorption from dilute solutions. 

 Some fresh-water animals, for example Eiihranchipus and Anguilla, seem to 

 depend exclusively, or almost so, on salt intake by food. In some insects there 

 are special organs for salt absorption, as the anal papillae in Ciilex and Chiron- 

 oimis larvae. In many other fresh-water animals the respiratory epithelia 

 contain secretory cells. 



Success on land (in air) requires protection against loss of water from the 

 body surface and from the respiratory membranes, and against excessive loss 

 of water by excretion. The only truly successful land animals are higher 

 arthropods (arachnids, myriapods, and insects) and higher vertebrates (rep- 

 tiles, birds, and mammals). All others are largely restricted to an environment 

 which is at least moist. Life in moist soil is osmotically similar to but less rigor- 

 ous than life in fresh water. The land arthropods are well protected against 

 surface loss of water by their chitinous coat. Many aquatic Crustacea also 

 iiave similar protection, and the entire group would perhaps succeed on land 

 il it solved the respiratory problem. Some crabs do spend hours or days on 



