ABERRANT FORMS OF LAMELLIBRANCHIATE GILLS. 605 
upon themselves at the free ends, the reflected portions 
lying on the outer side of the outer, and on the inner side 
of the inner, series of filaments . . . (Mytilus Pecten). 
In most Lamellibranchs, the gills are four elongated plates, 
each of which is in fact a long narrow pouch, with its open 
end turned toward the hemal face of the body ” (‘ Inverte- 
brates,’ pp. 408-9, Am. Ed.). My own observations lead me 
to the same conclusion. In fact, it is difficult to see how 
the pouch-like gills of Unzo can give rise to such forms of 
branchie as are found in Nucula and Yoldia. By a very 
circuitous route they may have degenerated into their 
present rudimentary state, it is true, but all recent observa- 
tions tend to show that while other organs in the Lamelli- 
branchiata have been steadily degenerating, the gills, on the 
contrary, have become highly developed and perform func- 
tions which the probable change of the animal from the 
motile to the sedentary habits of life has forced on these 
gills. If, then, there has been no considerable degeneration, 
and if the homologies of different parts of these branchiz 
are, as I have stated above, the filamentary character of the 
primitive Lamellibranchiate gill is placed beyond doubt. 
I believe further light is thrown on the subject by the 
gills of Nucula and Yoldia. Peck shows that the gills 
primarily consisted of a series of filaments, but does not 
attempt to account for the fact that these filaments have 
come out in long rows on the side of the body. I venture 
to suggest an explanation. If we reflect for a moment, I 
think we shall see that the gills of Nwcula and Yoldia may 
be considered as a stem which, being folded on either side 
to increase the surface of contact with the water, gives rise 
to the flat plates which I have homologized with the de- 
scending limb of the gill-filament of Myti/us and other like 
forms. The plates are, strictly speaking, nothing but the 
epithelial covering of the stem raised into folds and enclos- 
ing between the two sides of the folds a blood-channel. In 
the case of Yoldia mesoblastic lacunar tissue is carried out 
into the folds. According to this theory, the gill of the 
Lamellibranchiata was originally a longitudinal ridge on the 
side of the body. Probably in this a blood-vessel ran, and 
must have served as the organ of respiration. In course of 
time, however, this ridge became folded for the increase of 
the surface of contact with the water and thus produced 
papilla on its two sides—rudiments of the future gill fila- 
ments. The gills of Nucula and Yoldia have gone but 
little beyond this stage. I think there is much to support 
this view. Stepanoff (loc. cit.) observed in Cyclas that the 
