20 Comparative Animal Physiology 



The eggs of fresh-water fish take up a httle water during the first 1 to 6 

 hours after oviposition, but thereafter they are impermeable to water.^^''' i''"* 

 Oxygen can penetrate eggs which are impermeable to water and salts. The 

 barrier which keeps water from entering the embryo is the vitelline membrane, 

 the shell or chorionic membrane being freely permeable. ^''^ -^^ Impermea- 

 bihty of the vitelline membrane is maintained unless the membrane is injured 

 or calcium removed from the medium. ^■' 



Eggs of Daphnia (data of Przylecki, quoted by Krogh^^'O and of Limnea 

 stagnalis-^' are permeable to water and swell considerably (45 per cent, 

 Limnea') during the first few hours after oviposition. Similarly frog eggs are 

 initially permeable to water and then much less permeable. The osmotic con- 

 centration falls from about 120 mM at the time of laying to 80 mM by the 

 time the blastopore closes, because of water uptake. ^-^^ In the tail-bud stage 

 osmotic concentration again increases, owing primarily to accumulation of 

 organic products. After hatching, the embryos can grow for a time by water 

 uptake; chloride absorption begins as soon as external gills are present. 



The exceedingly low or negligible permeability of the vitelline membrane 

 of many eggs protects the embryos against osmotic stresses until normal osmo- 

 regulatory mechanisms can function. 



AN INVASION OF FRESH WATER, LAND, AND SALT LAKES: 

 CRUSTACEA 



Crustacea occur in water of a wide range of salinities, from the most dilute 

 fresh-water ponds, through all degrees of brackishness, to the ocean, which is 

 a 3.5 per cent salt solution, and even in salt lakes of some 22 per cent salt con- 

 centration. Crustacea are of marine origin and have invaded fresh water at 

 numerous times and places. In fact, some forms seem to be on their way from 

 ocean to fresh water at present. By comparing Crustacea of different osmotic 

 capacities it is possible to picture the changes which have made possible their 

 distribution and to predict which groups are likely to venture into new con- 

 ditions of salinity. 



The Crustacea can be arranged in a series with respect to osmotic capacities. 

 The following seven groups are distinguished by their habitats and the respon- 

 ses of their blood concentration to changes in the concentration of the medium 

 (Figs. 10, 12, 13, 14): 



1. Animals limited to sea water, sometimes called stenohaline;"^ they show 

 no osmoregulation when put into dilute sea water, but they do show volume 

 regulation. Example: Alaja. 



2. Crabs which regulate their osmotic concentration to a limited degree and 

 are therefore able to venture into slightly brackish water. Examples: Cancer 

 and Eriphia. 



3. Animals which regulate better in dilute sea water and are common in 

 regions of the shore near where fresh-water streams empty (euryhaline'^). 

 Their body fluids are adjusted to the medium in high concentrations and 

 become regulated by remaining hypertonic in low concentrations. Examples: 

 Carcinus and Hemigrapsiis. 



* The terms stcnohaline and euryhaline refer, respectively, to animals tolerating only 

 slight changes in sahnity and to animals tolerating considerable dilution. Since all degrees 

 of intcrgradation occur, the terms are of httle practical value. 



