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and ventral sides of the gill. The outer walls have nuclei, hut tliey occur at widely separated 

 intervals. 



The bypodermic pillars cross the blood Chambers. Wbere tbey occur the walls of the 

 Chambers curve round them, forming sheaths. Thus the cavities containing blood are everywhere 

 separated from the hypoderm by a boundary wall. 



It is Seen (Fig. 12) that the dorsal Chamber becomes very wide where the general part of 

 the gill adjoins the special part. This enlarged portion of the dorsal chamber is (as seen in section) 

 the main excurreut blood-channel. By examining a series of sections, proceeding from the one 

 figured toward the proximal side of the gill, it is seen that the rest of the dorsal chamber becomes 

 gradually narrower and finally disappears, while the enlarged portion continues to the union of the 

 gill witli the basal Joint. The gradual narrowing of the dorsal chamber (apart from the portion 

 forming the excui'rent channel) is correllated with the gradual widening, toward the proximal side 

 of the gill, of the ventral chamber. Finally, where the dorsal chamber disappears the greatly en- 

 larged ventral chamber becomes the main incurrent channel. 



These relations may be described in other words by saying that the partition separating 

 the dorsal and ventral Chambers gradually approaches, in the direction of the proximal side of the 

 giU, the outer wall of the dorsal chamber and finally becomes in contact with it, excepting along 

 the region where the general part of the gill adjoins the special part. 



By examining a complete series of sections, proceeding from the one figured to the distal 

 side of the gill, it will be found that the partition separating the dorsal and ventral Chambers comes 

 to an end and that the end lies free in the cavity of the gill (excepting as supported by the pil- 

 lars). This results in a communication of the dorsal and ventral Chambers in the distal region 

 of the gill. 



The structure and relations of the special part of the gill. The dorsal and 

 ventral blood-chambers of the general part of the gill are continued into the special part of the 

 gill. But in the special part there is present, in addition, a third blood-chamber which lies between 

 the other two (Fig. 13, d.b.c, m.b.c, v.b.c). This middle chamber is formed by the forking of 

 the partition separating the dorsal and ventral Chambers in the general part (Fig. 12, Fk.). Each 

 brauch of the fork retains the structiu-e of the main partition; that is to say, consists of a double 

 wall, with nuclei lying in the narrow intervening space. While the dorsal and ventral Chambers 

 are thus separated along their lateral faces thoy communicate with each other in certain places at 

 the margin of the gill. This is shown in Fig. 13 which represents a portion of a section of the 

 gill taken in the direction of the line rr', Fig. 11. It is seen that communication is afforded through 

 the continuety of the outer boundary wall of the dorsal blood-chamber with the correspouding wall 

 of the ventral chamber. At these places of communication of the two outer Chambers the middle 

 Chamber ends blindly. I have not been able to satisfactorily demonstrate places of communication 

 between the middle chamber and the other two. But since the middle chamber contains blood 

 with corpuscles, I conclude there are such Communications; further, as the middle chamber is blind 

 at its inner end (lying between the branches of the fork) these Communications must be along the 

 margin of the gill. 



In addition to the three blood-chambers in the special part of the gill there are other 

 Chambers which contain air. These are two in number, situated one on tlie dorsal and one on the 

 ventral side of the gill. The dorsal air-chamber is bounded outwards by the wall of the gül and 



