and ventral sides of the gill. The outer walls have nuclei, but 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 enlarged
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 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 pillars).
This results in a communication of the dorsal and ventral chambers in the distal region
of the gill.
T h e s tr u c tu r e and r e la t io n s of th e s p e c ia l p a r t o f th e g ill. 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
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 portion of a section of the
gill taken in the direction of the line i t ' , 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
inwards by the outer wall of the dorsal blood-chamber (Fig. 13, d.a.c.). The ventral air-chamber
has corresponding relations (v.a.c.). In other words, in the special part of the gill there is a space
between the general wall of the gill (composed of the chitine and hypoderm) and the boundary wall
of the blood-cavity. and this space contains air. This space does not communicate by any opening
with the outside of the gill.
I find that in many preparations of sections the hypoderm does not appear to be present
as a layer lining the chitine in the special part of the gill (excepting at the free end of the gill).
I am inclined to the opinion that the hypoderm may become withdrawn from the chitine and centered
in the pillars. As already observed, in this region of the gill the hypodermic nuclei are found
only as associated with the pillars. It would seem possible that such a withdrawal of the hypoderm
may take place periodically with moulting as soon as a new chitinous layer has been formed.
The hypodermic pillars in the special part of the gill are arranged in rows which divide
the chambers into a series of compartments separated by broken walls. The appearance of these
compartments, as seen when the gill is examined from without has already been referred to, p. 47,
as being a series of radiating blood channels communicating with the main excurrent channel. It
will be understood, however, that the compartments of the dorsal blood-chamber only open directly
into the main excurrent channel.
P h y s io lo g y of th e outer g ill. The co u r se o f th e c ir c u la tio n of th e blood.
The course of the circulation of the blood in the outer gills of Oniscus can be seen by removing
the first two pairs of thoracic legs, placing the animal in water in a watch-glass, putting on a cover-
glass, and examining under a low power of the microscope. Fig. 14 is based upon examinations
thus made. The dotted arrows represent the courses taken by the in-flowing or venous blood and
the lined arrows the courses taken by the out-flowing or arterial blood. The blood, entering the
gill at the basal joint, for the most part takes the path of the main incurrent channel. But as
the inner boundary of this channel is a broken wall (formed by a row of hypodermic pillars) the
blood constantly escapes from the channel and spreads over the entire region of the general part
of the gill. This distribution of the blood further depends upon the curvature of the outer wall
of the main in-current channel (the marginal wall of the gill) from which the current of blood is
constantly being reflected.
That portion of the blood which thus makes its way to the distal region of the gill then
turns in its course and enters the main excurrent channel. The rest of the blood passes from the
general to the special part of the gill, entering the radiating channels and proceeding directly outwards
toward the margin of the latter portion. Here it turns in its course and, following the radiate
channels but in the reverse direction, enters the main excurrent channel. A small portion
of the blood, however, does not enter this channel but joins the outflowing stream directly at the
basal jointV
A portion of the blood, entering at the basal joint, does not go to the general part of the
gill but to the special part and appears to circulate as follows: It tends to spread over the region
of the special part but mingling with the currents flowing from the general to the special part,
its course is gradually lost in these currents.
If now we apply the facts of structure and relation gained by the study of the anatomy
of the gill to the explanation of the course of the circulation as observed from without, and above
described, we are led to the following results: