16 
surface of exposure, and second, a thinning of the same wall, in order to secure the permeability 
requisite for a ready exchange of elements between the two media. 
But modifications of structure in each of these two directions are subject to limitations 
imposed by other mechanical conditions. Increase of surface is limited by the requirement of a 
moderate and proportionate size of the part; and the thinning out of the wall is limited by the 
requirement of strength. 
Applying these principles we see that in the specially modified portion of the chitinous wall 
at the exterior of the gill increase of surface and at the same time thiness of wall is secured through 
the fill-rowing of the chitine. Furthermore, the furrowed structure is that which in the least degree 
compatilde with the attainment of these two ends detracts from the strength of the wall. The 
arrangment of the furrows in a net-work is adapted to securing the largest linear extent of grooving 
and at the same time the least sacrifice of strength of the wall. The shape of the furrows which, 
as seen in section, (Fig. 3 a, gr.) is that of an oval with the long axis at right angles to the face 
of the wall is, I conceive, adapted to a three-fold purpose, namely : first, to containing a large 
amount of air relative to the space occupied; second, to retaining this air in the groove (l>y means 
of the narrowing of the opening); and third, to reduce to the least extent the strength of the wall 
(by means of the resistance to fracture secured through the curved surface of the oval). 
In spite of these adaptations, it is evident that the grooved portion of the wall of chitine 
has not the same strength as the rest of the wall. As a compensation for this the marginal region 
of the grooved area is depressed lielow the level of the plain area and is hounded by a ridge of 
chitine (Rd. Fig. 4) formed by a thickening of the wall at the line of junction of the two areas. 
This ridge is the curved line, convex toward the middle of the gill, mentioned above in 
the description of the appearance of the inner face of the gill. Furthermore, the grooved area as 
a whole is hounded by convex surfaces — a means of increasing its strength. 
In the tree extent of surface is secured by the folding of the wall and permeability by the 
thinning out of the same. The protected position of the tree inside the gill permitts a high degree 
of tenuousness of the wall. The tubular form of the branches, as well as their rounded form at 
the ends, afford the greatest degree of strength consistent with their extreme thinness. 
The tree, as a mass, is sonienhat spherical in shape (Fig. 1), which secures the most favo- 
rable distribution of the branches in the fluid surrounding them. But the sphere is depressed on 
the basal side of the tree, leaving a space between the base and the grooved area of wall of the 
gill, as noted above (p. 22, see also Fig. 4, B. ch.). It is evident that this is an adaptation for 
bringing a relatively large amount of blood into relation with the air contained both in the grooves 
and in the basal branches of the tree. 
The circulation of the blood in the gill. From my observations I am lead to infer 
that all of the blood in the course of its circulation through the gill passes through the space or 
chamber, just referred to. By mounting a living animal in water the movements of the blood cor- 
puscles within the gill can in part be observed. The blood enters the gill at its connection with 
the basal joint and is first distributed through the cavity of the general part of the gill. Due to 
the position of the gill in relation to the joint the course of the current is directed toward the 
inner and lower sides of the blood cavity. But owing to the curvature of the inner side of the 
cavity (corresponding to the curved outline of the side of the gill lying toward the middle line of 
the body) the blood particles are swerved in their course toward the special part of the gill. They 
