Water 27 



VA hours. Carcinus took up less than one-seventh as much iodine as Cancer 

 through a similar area of the dorsal carapace. '" 



The urine of Carcinus may be hypertonic, hypotonic, or isotonic to the 

 blood when in sea water, and when in dilute sea water it may be slightly 

 hypotonic to the blood but more concentrated than the medium.'^'' '■'•' The 

 high urine output in normal sea water cannot be osmotic water but rather 

 appears to be a mechanism of ionic regulation, and water must be taken in 

 non-osmotically when the crabs are isotonic with the medium. Water is 

 necessary for the excretion of undesirable ions like Mg+ + and SO4 \ -^^' 

 It is probable, therefore, that the kidneys of marine Crustacea developed as 

 salt-regulating mechanisms. 



In a dilute medium Carcinus consumes considerably more oxygen than in 

 normal sea water, in contrast to Maja.^^^- --^ Part of the increased energy 

 must go toward maintenance of the high concentration gradient. \ he con- 

 centration gradient is maintained partly by active salt absorption. '^^ Starved 

 specimens maintain their concentration of body salts at a constant level despite 

 continued urine excretion. Also, when transferred from a medium of low to 

 one of higher salinity, the crabs take up chloride even against a gradient to 

 their more concentrated blood. For example, when they were transferred from 

 a medium with 8.57 mg. Cl/ml. to one with 11.45 mg. Cl/ml., their blood 

 concentration increased from 12.0 to 14 mg./ml. The transfer and uptake 

 took place with the digestive tract closed, hence the gills were probably actively 

 absorbing chloride. ^^'^ 



Three mechanisms play a part in osmoregulation of Carcinus in dilute 

 medium: low permeability to both water and salts, increased fluid output with 

 increased osmotic gradient, and active salt absorption. It is impossible to 

 separate osmotic regulation from ionic regulation except by degree, in different 

 media. 



It is likely that Rhithropanopeiis harrisii is similar to Carcinus in osmotic 

 regulation (Fig. 13, curve C). '-- 



Regulation in Both Hypertonic and Hypotonic Media. In Pachygrapsus 

 crassipes (Fig. 13, curve A), Leptograpsiis variegatus,*'-^ Sesarma erythro- 

 dactyla, ^^~ Heloecius cordiformis (Fig. 14, curve A), ''^ Ilea crenulata 

 (Fig. 14, curve B), ^-- Palaemonetes varians var. microgenita (Fig. 14, curve 

 F), 1^^ and Leander serratus (Fig. 14, curve D), ^''^ the blood concentration 

 is even more constant with changes in the medium than it is in Carcinus. 

 These crabs remain hypotonic in a concentrated medium. Pachygrapsus, 

 which lives in sea water near the shore, is normally more dilute than sea 

 water.--' ^-- Some crabs, such as Pachygrapsus inannoratus, leave the water 

 many times a day. Palaemonetes varians var. microgenita can live in brack- 

 ish, salt, or nearly fresh water, and Crago vidgaris lives in the Elbe at a salinity 

 of 0.43 per cent. -^- 



Curves relating internal to external concentration in these animals arc nearly 

 flat, with hypertonicity of the animal in dilute medium and hypotonicity in 

 concentrated medium (Fig. 14). ''''' At extreme dilutions a rapid fall in blood 

 concentration occurs and the condition of the animal is poor. 1 he maximum 

 concentration at which the downward flexion of the Ai/A., curve occurs 

 must limit the freshness of water which can be entered by the crabs. Investiga- 

 tion of the means by which osmoregulation thus breaks down at low concen- 



