424 
bulletin of the bureau of fisheries. 
fuse and their cavities intercommunicate, forming a sort of irregular sinus. Here all 
the vessels of the two lamellae seem to be in open communication, but the starch mass 
was not forced into them sufficiently to show this. If one can rely upon sections, this 
is true. Blood which does not find its way across to the direct lamella before reaching 
the edge of the gill does so here by passing through one of the very numerous transverse 
vessels which are present in this region. As has already been said, these intercommuni- 
cate, and this may be an adaptation to take care of the extra amount of blood which 
flows through the gills during muscular activity or when the heart beats rapidly from 
any cause, supposing that the capillaries are not sufficient to accommodate it at such 
times. Only a very small part of the blood passes around the edge of the gill. As 
has been stated already, the filaments disappear at the edge of the gill, but it is possible 
to trace vessels to the edge where they communicate with irregular spaces which pass 
around to the opposite lamella. 
It is clear that the blood does not pass, as a whole, down one lamella and up the 
other, as is the case in Pecten tenuico status , as described by Drew (2). All the vessels of 
the outer lamella of this form are afferent, and all of those of the inner lamella are 
efferent. He found none ending blindly and no cross connections. On the other hand, 
Johnstone (6), studying Cardium edrde, found that half of the vessels of each lamella 
are afferent and half are efferent. He implies that the efferent vessels of each lamella 
open separately into the main efferent vein, but he does not make this plain, and his 
figures 24 and 30 are inconsistent. If we imagine the efferent vessels of the outer lamella 
as ending blindly above, and give them many cross connections with those of the oppo- 
site lamella, we have practically the arrangement found in Atrina, although the gills 
of the two forms differ considerably in other respects. As regards circulation the gill 
of Atrina is therefore intermediate between those of Cardium and Pecten, but is more 
nearly like CardiumA 
RESPIRATORY CURRENT. 
The respiratory current in Atrina is remarkably strong. When specimens are as 
much as 6 inches below the surface a very considerable agitation of the water directly 
above them is perceptible when the respiratory current is running full force. In fact, 
the water fairly boils. The mantle, being open, may admit water at any point ventral 
to the inter mantle septum and it is expelled dorsal to this structure. While the respira- 
tory current is flowing the edges of the mantle are brought quite close together, so that 
objects of any considerable size are prevented from entering the mantle chamber. It 
was found difficult to get admission even for powdered carmine. The sensory tentacles 
detect solid objects in the water and the mantle closes, preventing their entrance. When 
one shell is partially removed and the mantle lobe folded back the respiratory current 
within the mantle chamber can sometimes be seen. Powdered carmine shows strong 
a Bonnet describes a different circulation for the gill of Pinna nobilis , a form so closely related to Atrina that one would expect 
to find no fundamental differences in the circulation. Menegaux finds the work of Bonnet incorrect. His description of the 
anatomy of the gill for Pinna agrees very closely with mine for Atrina. but*he gives a different description of the course -of the 
circulation through it. I am inclined to think they are the same. 
