BY R. J. TILLYAKD. 403 



CO .J in the trachea? must eventually be replaced almost completely 

 by a mixture of O and N" closely resembling air. (As the larva 

 must be continually using up the O, it is not necessary to assume 

 that the total amount of gas in the trachepe at any given moment, 

 if analysed, must have the exact composition of air). The rate 

 at which the exchange takes place will be governed bj' the mor- 

 phological coiulitions in the rectum. As the expanse uf the 

 rectal chamber is limited, only a certain amount of aei'ated water 

 can be drawn in at one time. Hence, if this water bathes an}' 

 given length of cylindrical tracheal tubing, it is cleai- that, the 

 smaller the bore of the tube, the quicker will the exchange of 

 gases take place. The rate of exchange of gas may be roughly 

 taken as inversely propoi'tional to the area of the cross-section of 

 the tube, i.e., to the square of the diameter of the tube. Also, 

 any increase in the length of tube exposed to the watei- will 

 facilitate the rate of exchange. Thus the ideal conditions 

 for a rapid e.rchatu/e are fidjilled in tlir case of a lan/e number 

 of e.vceediuyl y Jini' tubes hatnny a c/reat portion of their lenyth 

 bathed by the water. This is exactly the morphological condition 

 developed in the rectal gills of Anisopterid larva\* 



Section 3. — The Nature and Action of the Cephalic Heart. 



Tn my previous study upon the larva of Anax papuensis {I.e.), 



T described the action of the peculiar pumping-organ of the head, 



and suggested for it the name of the cephalic heart. Owing to 



the rapidity of the act of hatching, and the fact that this organ 



* Note. — In a letter received from Mr. G. L. Purser, B. A., of Trinity 

 College, Cambridge, in which he di.scusses my papei' on the Phj^siology of 

 the Rectal Gills, lie remarks on the peculiar problem of tlie first appearance 

 of gas in the trachea^: — "I don"t see why the gas slioald appear just where 

 it does. My idea is this : the gas appears first in the thickest part of the 

 embryo; it appears just when the larva is making a series of strong 

 muscular efforts for the first time. Why, therefore, should it not be CO., ? 

 Tiie gas appears wliere COo will be most concentrated; it appears mlien it 

 is most concentrated." This letter was received only a few days after my 

 expei'iments were completed, and was, therefore, written some time before 



them. I desire, therefore, to give Mr. Purser due credit for the first eou- ^ -^ 



ception of the idea stated in liis letter. — R.J.T. /vNV3' ^ A ^ 



Lul LI 8 R A F 



