WATER, SALTS AND MINERALS 



35 



takes place. Nevertheless, there is some regulation of volume. Specimens 

 of Dendrostoma immersed in slightly dilute sea water (90%) swell, but 

 after 24 hours they lose weight and return to normal (Fig. 2.4). Volume 

 control is also shown in concentrated media (120% sea water). Critical 

 studies reveal that the body wall of Dendrostoma is highly permeable to 

 water, but only slightly permeable to salts; permeability to water and salts 

 is greater inwards than outwards. Other loci, gut or nephridiopores, how- 

 ever, are more permeable to salt. The increase in volume which occurs 



2 3 4 s 



Time (days) 



Fig. 2.4. Volume Control Exhibited by Golfingia gouldii in 

 Moderately Dilute Sea Water (75 % and 90 %) 



Loss of weight of worms (controls) in normal sea water shown for comparison 

 (Simplified from a diagram of Gross (41).) 



when worms are placed in hypo-osmotic media is due to passive diffusion 

 of water across the semipermeable body wall ; subsequent volume regula- 

 tion is achieved by loss of salt through nephridiopores and outward diffu- 

 sion of water. In addition to these processes, there is some release of 

 osmotically active particles into the blood from the body wall, tending to 

 counterbalance the loss of salts to the exterior (41). 



Other poikilosmotic marine invertebrates which take up water and 

 swell in dilute sea water are Nereis pelagica, Perinereis cultrifera, Arenicola 

 marina and Sabella pavonina (Polychaeta) ; Mytilus edulis and "Doris" 

 (Mollusca); and Caudina chilensis (Holothuria) (Fig. 2.5) (28). Some of 



