26 
and ventral sides of the gill. The outer walls have nuclei, hut they occur at widely separated 
intervals. 
The hypodermic pillars cross the blood chambers. Where they 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 excurrent 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 with the basal joint. The gradual narrowing of the dorsal chamber (apart from the portion 
forming the excurrent 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 
gill, 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 he 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 commxinication 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 pai’t (Fig. 12, Fk.). Each 
branch of the fork retains the structure 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 they communicate with each other in certain places at 
the margin of the gill. This is shown in Fig. 13 which represents a ijortion 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 corresponding 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 the dorsal and one on the 
ventral side of the gill. The dorsal air-chamber is bounded outwards by the wall of the gill and 
