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The blood enters the ventral blood-chamber and spreads out through the entire cavity. 

 In tlie distal part of the gill where the partition separating the ventral from the dorsal chamher 

 comes to an end the blood passes into the dorsal blood-chamber and immediately into that portiou 

 of it tbrming the main excurrent channel. The 1)1(H)(1 entering the ventral Chamber of the special 

 part (jf the gill flows through the conipartments of that chamher to the places where they commu- 

 nicate with the corresponding compartments of the dorsal chamljer and there enters the latter. It 

 flows through these to the main exciu-rent channel, excepting a, small portion which passes into 

 the excurrent channel at the Inisal Joint. 



The function of the blood- Chambers. The function of the ventral chamber is ob- 

 viously to receive venous blood from the body and distribute it over the entii-e outer face of the 

 gill. It is there separated from the external atmosphere by its own wall and the ventral wall of 

 the gill. As this wall of the gill is rather thin it would appear that an exchange of gases may 

 take place between the blood in the ventral chamber and the external air. (This statement is in- 

 tended to apply here only to the general part of the gill; the exchange of gases in the special part 

 is considered below.) The function of the dorsal chamber is evidently to expose the blood still 

 fui'ther to the action of air (as described below) and finally to collect the pm'ified blood into a 

 Channel through which it passes Ijack to the body. The use of the middle chamber appears to be 

 passive rather than active. The space taken up by it reduces by so mucli the depth of the 

 Chambers above and below and thus seciu-es the exposui-e of a very thin stream of blood to the 

 action of the air in the air-chambers. 



The function of the air-chambers. When the blood euters the ventral chamber of 

 the special part of the gill it is sepai-ated from the air in the corresponding air chamber only by 

 the Single thin memlnane which forms the outer boundary wall of the blood-chamber. The ordinary 

 conditions of the respiration of air are thus satistied. As the blood passes at the margin of the 

 gill into the dorsal chamber it is further exposed to the action of air in the dorsal air-chamber 

 and the respiratory process is here completed. 



The air-chambers, as already noted, do not communicate by openings with the exterior of 

 the gill. The contained air must therefore enter by penetration through the wall of the gill. The 

 wall, as we have seen, is here very thin, the hypoderm, as a rule, appearing to be withdrawn and 

 leaving only a very thin layer of chitine. 



As has already been stated, when an animal is placed in water the appearance of aii- in 

 the gills passes away. This Observation led to a number of experiments the chief pui'pose of which 

 was to gain evidence, in addition to the evidence derived from direct insi)ection and from the study 

 of the structm-e of the gill, that air is normally present in the Chambers.' 



1 . An animal was placed in water until the appearance of aii- in the gills had passed away. 

 It was then killed and the tissues fixed by bot 33 "/o alcohol. In sections of the gills prepared 

 from this specimen the aü--chambers did not appear empty (as usual) but contained blood without 

 corpuscles, that is, blood plasma (appearing as granulated matter). 



This experiment having indicated that under the conditions imposed the air was replaced, 

 in part, at least by plasma of blood, we next sought to modify the experiment in order to ascertain 

 whether the presence of blood in the Chambers was due entirely to the conditions being artificial. 



' These experiments were suggested to me by Dr. zur Strassen. 



