— 16 — 



surface of exposure, and second, a thinning of the samc wall, in order to secure the j^ermeability 

 requisite for a ready exchange of elements between the two media. 



But modifications of stnicture in each of tliese two directions are subject to limitations 

 imposed by othev mechanical conditions. Increase of siu'face is limited l)y the requirement of a 

 modei-ate 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 portiou of the chitinous w all 

 at the exterior of the gill increase of surface and at the same time thiness of wall is seeured through 

 the furrowing of the chitine. Furthermore, the furrowed structure is that which in the least degree 

 compatible 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 secui-ing 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 purpoie, naniely: first, to containing a large 

 amount of air relative to the space occupied; second, to retaining this air in the groove (liy 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 seeured 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 f(ir this the marginal region 

 of the grooved area is depressed below the level of the piain area and is bounded l)y 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 bounded l)y convex surfaces — a means of increasing its strength. 



In the tree extent of surface is seeured 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 tuljular 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 somenhat spherical in shape (Fig. 1), which secures the most favo- 

 i-able distribution of 4.he 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. eh.). 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 refeiTcd to. By mounting a living animal in water the movements of the blood cor- 

 puscles within the gill can in part be observed. The ])lood 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 (correspouding 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 pnrt of the gill. They 



