BY R. .1. TILLYARD. 401 



pearf-d to suffer little inconvenience oi' irritation, tliouyh the 

 liquid was repeatedly sucked into the rectum. In both larva?, 

 the main tracheal system remained intact for fioehi' hrntrs (a 

 very strikin,y lesult, cf. Control Experiment). In the younger 

 larva, the rectal capillaries remained absolutely intact at the end 

 of tNselve hours (again, <■/. Control Experiment). In the older 

 larva, the rectal capillaries collapsed partially at tlie end of three 

 hours. 



Conclnttiou. — The action of the Azol is clearly resisted by 

 chitin, and hence, no safe conclusions can be drawn as to what 

 gas was present in the main tracheal system. In the rectal 

 capillaries, there was little or no oxygen present in the case of 

 the younger larva, but enough oxygen was present in the case of 

 tlie older larva to affect the stability of these tubes when it was 

 withdrawn by the Azol. 



Experiment 4. — A moribund larva, taken fi'om Series A, soon 

 after hatching, was placed in a solution of 4% NaOH. This 

 larva remained inert, without sucking the solution into its 

 rectum, and died within an hour. lifKiilt, same as stated for 

 the larvte in the Control Experiment. 



Conclusion. — No guide as to composition of gas. Either the 

 solution was too weak to penetrate the chitin, or else the fact 

 that the larva was moi'ibund, and did not draw the solution 

 into its rectum, prevented contact between the solution and the 

 most permeable part of the animal {i.e., the thin, chitinous cuticle 

 of the rectal gills). 



Besides the larvae used in the above experiments, a single 

 larva emerged from Series B on April 9th, at a time when I was 

 unable to make use of it, and two moribund larv?e emerged from 

 Series A on April 10th, dying before I could make use of them. 

 No more larvai emerged, and no less than sixteen dead embryos 

 remained inside eggs of Series A until April 20th, when they 

 were thrown away. 



The results of my experiments, few though tliey were in 

 number, appear to point definitely in the following direction: — 

 While there is no appreciable quantity of CO.^ in the tracheal 



34 



