RESPIRATION IN WATER 



25 



forms there is no regular circulation, but the coelomic fluid is 

 kept in motion by cilia. Further in a number of transparent 

 pelagic forms belonging to many different groups (e.g., larvae 

 of Crustacea and insects). 



In a number of fish em- 

 bryos and larvae, circulation 

 develops before the respira- 

 tory organs proper, and part 

 of the surface functions pro- 

 visionally as a respiratory 

 organ and is provided with a 

 capillary (Fig. 4) or lacunar 

 (Fig. 5) system of blood ves- 

 sels. In the young of Acara 

 (a fish belonging to the Chro- 

 mida) the tail is vascularized 

 and functions as a respira- 

 tory organ. 



In several cases it may 

 be difficult to find out whether the whole of the surface 

 is permeable to oxygen or whether special appendages 

 function specifically as respiratory organs. H. M. Fox 



Fig. 4. Fish embryo {Xiphophorus) 

 with blood-vessels on surface of yolk sac. 

 Size probably a couple of mm. (Kry- 

 zanowski.) 



Fig. 5. Pike larva 6-7 mm long. Blood lacunae in yolk sac. (Kryzanowski.) 



(1921) introduced the use of certain flagellates (Bodo) for 

 this purpose, and certain luminous bacteria {Bac. phosphores- 

 cens) can also be used. Thorpe (1932) describes the culture of 

 the bacilli which require a rather high salt concentration, 

 and also of the flagellate Polytoma which he has found specially 



