WATER, SALTS AND MINERALS 57 



for the normal physiological requirements of the animal. Marine mam- 

 mals which feed largely or exclusively upon invertebrates are subject to 

 an additional osmotic strain. The salt content of the urine is generally less 

 than 32 g/1., and accurate experiments on the dolphin Tursiops truncatus 

 have shown that individuals of this species fail to regain their original state 

 in 9 hours after administering 2 1. of 0-5 m NaCl. It is still uncertain how the 

 water requirements of marine mammals which feed largely upon inverte- 

 brates are met. Some workers have suggested that water can be absorbed 

 by the mucous membranes of mouth and gullet. The fat content of the 

 food of these animals (copepods, herring, etc.) is high (Table 2.1), and 

 could provide a rich source of metabolic water (30, 31, 77). 



Osmotic Relations among Parasites. Parasitic animals are sometimes 

 subjected to peculiar osmotic conditions as the result of their parasitic 

 mode of life. These arise in relation to the external medium of the host 

 (ectoparasites and free-living stages), the internal medium of the host 

 (endoparasites) and the use of the host's fluids as a source of food (blood- 

 sucking ectoparasites). Osmotic relations have been investigated in only a 

 few species and information is limited, but from what is known of their 

 life-histories parasites may be expected to display peculiar conditions of 

 adaptation and regulation in relation to their biotic environments. 



Highly specialized parasites, which have external phases and utilize a 

 succession of hosts, may be exposed to widely different salt concentrations 

 at different stages of their life-history. Examples are marine trematodes 

 passing from marine invertebrates to vertebrate hosts. These soft-bodied 

 animals appear to be poikilosmotic and osmolabile. Some marine cercariae 

 will withstand sea water diluted to 50-25%. At least one species, Spelo- 

 trema, is restricted in distribution by salinities below 17% (7). 



Cestodes from fish appear to be isotonic or slightly hypotonic to the 

 hosts' fluids. Plerocercoid larvae of Schistocephalus from Gasterosteus are 

 poikilosmotic, and shrink or swell when the external medium is made 

 more concentrated or dilute (above or below 0-75% NaCl). Another 

 cestode, Bothriocephalus, from the spiral valve of Scyliorhinus, is slightly 

 hypotonic to the surrounding intestinal fluid (A 2 against A 2-08-2-65). 

 This is an interesting problem since a large part of the osmotic pressure of 

 selachian body (and intestinal) fluids is due to urea (p. 51). Evidence 

 exists that intestinal nematodes possess some powers of osmotic and ionic 

 regulation (49, 131). 



Some parasites have become adapted to host animals which migrate 

 into estuaries, fresh water or even upon land. Species of fish-lice Argulus 

 infect both fresh- and salt-water fishes and will tolerate immediate transfer 

 from one medium to the other. Some bopyrid and rhizocephalan parasites 

 accompany their crustacean hosts into brackish or fresh water. In contrast 

 are caligid copepods which are parasitic upon sea-run trout and salmon: 

 these parasites are stenohaline, and are killed when the host returns to 

 fresh water (2, 146). 



Ectoparasites, which infect terrestrial crabs, face a somewhat different 



