ANATOMY AND PHYSIOLOGY OF ATRINA RIGIDA. 
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a large modified filament, or as a single filament with its subsequent development of 
subfilamentar lacunar tissue, and there is some evidence that this is correct, viz, the 
epithelium lining the ciliated groove is continuous with that of the blood vessel. (Fig. 
3, e.) At the edge of the gill also the resemblance becomes much more striking where 
it assumes clearly the appearance of a filament. 
STRUCTURE OF THE FILAMENTS. 
The structure of the individual filaments is best made out in cross sections such as 
that represented in figure 5. Each is composed of a simple epithelium which is lined 
by a very thin layer of chitinous material resembling a cuticle. (Fig. 5,c). Peck (12) 
considers this lining cuticle to be modified lacunar tissue. Sometimes protoplasmic 
Fig. 4.— Diagrammatic drawing of a bit of the gill, b, Interlamellar connectives; f filaments; i, interfila- 
mentar connectives; v and v ' , large blood vessels. The arrows indicate the direction of the flow of the 
blood. 
corpuscles can be seen lying upon its inner surface but none have been detected within 
it. There are no transverse bridges of this material such as are uniformly present in 
the filaments of the lower forms (Area, Mytilus, and Pecten). It has commonly been 
supposed that the septum in these forms divides the cavity of the filament into two 
blood channels — the one afferent the other efferent — and this view seems well founded. 
Drew (2) by use of injections found that in Pecten this bridge had no such physiological 
significance. He has therefore suggested that it may serve to prevent the walls of the 
filament from spreading under the pressure of the inclosed blood which might close the 
incurrent ostea of the gill. He thinks that further study of gills of similar structure 
might throw light upon this interesting point. The gill of Atrina is made up on 
exactly the same plan as regards the shape of the filaments and amount and kind of 
interfilamentar connectives and yet there is no septum dividing the blood channel 
