ANATOMY AND PHYSIOLOGY OF ATRINA RIGIDA. 
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after being aerated in the gills. The latter is a T-shaped vessel, one arm of which lies 
in the suspensory membrane and carries blood from the anterior half of the gills, while 
the other arm lies immediately below the vessel X and collects the blood from the poste- 
rior portions of the gills. The two arms of the vessel flow together just anterior to the 
kidney and form a rather wide tube disposed at right angles to them. (Fig. 9, a.) 
This tube is perhaps an inch and a half in length and connects directly with the auricle. 
From these connections it is clear that the blood enters the gills through the 
reflexed lamellae and leaves them through the direct. When a starch mass was injected 
into the vessel X the afferent vessels of this gill were injected and the course of the 
blood was made out with certainty. The mass first distends vessel X and then passes 
to Y, through the interlamellar septa, filling it from end to end. The mass now enters 
the vessels of the gill which communicate with vessel Y and passes toward the gill’s 
free margin. (See fig. 20, pi. L.) Half of the vessels of this reflexed lamella 
are thus filled. Some of the mass flows across to the opposite lamella through the 
interlamellar connections and fills half of its vessels. (Figs. 3 and 4, b.) Examination 
shows that only those which end blindly above are filled with the mass, so that none 
of the injection mass finds its way into the efferent vessels of the gills. By injecting 
through the auricle, or the T-shaped vein which carries the blood from the gills, it is 
possible to fill all the vessels of the gills not already filled by injecting from the kidney. 
The mass first enters the direct lamellae and spreads across to the other. I have a 
preparation in which the afferent vessels are injected with a black mass and the efferent 
with a yellow one, which brings out the relationship between them quite clearly. It 
is evident that provision is made for making the blood pass through the smaller vessels 
of the gill before returning to the heart. 
We may conclude from evidence obtained from the injections and anatomical 
studies that the blood enters the gill through every alternate vessel of the reflexed 
lamellae, from which part of it spreads to the right and left in the interfilamentar connec- 
tives and filaments (fig. 4, i and /), finally finding its way into the neighboring vessels 
of the same lamella. These vessels (fig. 4, v') end blindly above so that it must yet 
pass across to the opposite lamella through the interlamellar connectives before finding 
its way out of the gill to the heart. On the other hand part of the blood on entering 
the gill passes at once across to the opposite lamella through the interlamellar connec- 
tives into vessels which end blindly above. From these it spreads laterally, right 
and left, in the interfilamentar connectives and filaments of this lamella and finally 
into the neighboring vessels which open freely above into the vein which leads back to 
the heart. The general course of the blood in the gill is therefore outward in the reflexed 
lamella and the opposite in the direct, and the vessels are so connected that it must pass 
through a capillary system in one lamella or the other before leaving the gills. 
The relationship of vessels just described holds good for all parts of the gills, except 
a narrow strip at their outer free margin. Here it is different and for completeness 
must be briefly described. Here the transverse vessels (interlamellar connectives), 
are very numerous and lie side by side. In the mid-line between the lamellae they 
