428 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
of this membrane now grows and turns upward. There is no mechanical bending of 
the filaments, but a growth upward of their fused ends. After the outer edge of this 
membrane formed by the fused ends of the filaments has grown upward for some dis- 
tance, the inner portion — that first formed — divides so as to mark out the ascending 
filaments as continuations of the descending ones. It is possible that this fusion of 
the ends of the descending filaments may not take place in the developing gills of 
forms like Area , where in the adult the contiguous filaments are nowhere fused with 
one another. 
Pelseneer (Ho. 17) now regards Nucula as the most primitive form. He believes 
Anomia and also the Arcidm to be directly descended from forms with plate gills. As 
I have said, these relations may be the true ones, and yet it seems difficult to explain 
the structure of the gills of the former in terms of those of the latter. One might 
much more easily suppose, a priori, that the latter had developed from the fi lamentous 
type, perhaps by the degeneration of the ascending portion of its filament. 
The point which I wish to make is shown in the accompanying cuts. Fig. 1 
represents two gill plates of one side of the body of Nucula. The shaded portion rep- 
resents the chitinous ventral edges. The ends of the plates here point downward and 
outward. Fig. 2 represents Pelseueer’s hypothetical type, in which the plates are 
much elongated ventrally and have become more like filaments. In such a series 
Pelseneer could not place Solenomya or Yoldia, but had to throw them out because 
their plates, instead of extending their outer ends downward, extended them in quite 
the opposite direction. 
How come the simplest filamentous gills, those of Area, Fig. 3. (I have figured 
the gills of Area pexata.) In this the ascending filament (ft/7) is fully formed, and the 
difference between this condition and that of Nucula is seen to be very great, iiotwith- 
standing this hypothetical form. 
It is possible that the ascending limb of the filament is a new structure which 
has suddenly developed in those forms most closely connected with the forms with 
plate gills ; if it is, however, merely a continuation outward of the descending filament, 
it seems as if we ought to regard the gill plate of Nucula as being homologous, not alone 
to the descending limb of a filament, as Mitsukuri has done, but to both descending 
and ascending limbs. 
afl 
