190 THE RESPIRATORY SYSTEM [CH. 



course of time, by a mixture of N and differing little, if 

 at all, from atmospheric air. Experiments which 1 have made 

 on newly-hatched larvae shew that equilibrium is practically 

 established in from one to three hours after hatching. 



Now the nitrogen is not required for the growing larva, 

 but the oxygen is being continually drawn upon. Thus the 

 partial pressure of the in the tracheal system is always 

 tending to diminish. 



This is the driving force for the extraction of further oxygen 

 from the water. The oxygen pressure must be kept up. For 

 every cubic millimetre of oxygen extracted from the tracheal 

 system for general use by the larvae, an equivalent volume 

 passes in through the syncytium to the capillaries. There is no 

 taking up of oxygen into chemical combination, as in the case of 

 red blood-corpuscles. The rectal syncytium is simply a diffusion 

 membrane, preserving the equilibrium of oxygen and nitrogen on 

 either side of it, and hence continually allowing the passage of 

 oxygen inwards from the water to the capillaries, while preventing 

 any increase in the amount of nitrogen. 



As regards the basal pads, these, I believe, act as supports for 

 the gills, and nothing else. In the Implicate Type of gill-basket, 

 where the arrangement is such that the gills support one another, 

 no basal pads are developed. In the case of the basal pad, the 

 advantage of fusion of the originally separate cells is that the 

 whole organ is enabled to act as a single cell. Its outer wall 

 becomes an osmotic membrane, allowing water to pass into the 

 pad until the requisite condition of turgescence is obtained. 



In conclusion, it is of interest to notice that the rectum of 

 Anisopterid larvae has a three-fold function (a) that of respiration, 

 (6) that of excreting the faecal pellets (as part of the hind-gut) 

 and (c) that of propulsion. By squirting the water out of the 

 branchial basket forcibly backwards, the larva propels itself for- 

 wards in a series of jerks. This is its usual method of progress. 



D. Caudal and Lateral Gills (figs. 35, 40-42, 84-86). 



Nearly all Zygopterid larvae possess three external caudal 

 gills. Of these, the median caudal gill is dorsal, unpaired, and 



