MORPHOLOGY OF L AMELLIB RAN C HI ATE MOLLUSKS. 
427 
It is possible that the oyster came, in its degeneration, through a form in many 
respects similar to Mytilus , from some active ancestor with a locomotor foot. Judg- 
ing from the most primitive of existing lamellibranchs — Nucula , Solenomya, etc. — the 
immediate ancestors of the group probably possessed a greatly developed foot. Ostrea , 
therefore, gives evidence of having departed much further from this ancestor in its 
degeneration than has Mytilus. This latter form, in turn, is less changed by its mode 
of life, and, noting here and there a form on the way to the most archaic, Venus, Area , 
Nucula, still less and less so. 
And yet the foot is too variable an organ to be made an exclusive means of classi- . 
fication or even the chief one. Anomia has lost the foot on account of its fixed habit, 
and yet it possesses some structures which indicate a more primitive position for the 
form than the one commonly supposed for it. Pelseneer goes so far as to place it 
immediately next the primitive forms with plate gills. 
But if there is one organ of lamellibranchs which is most subject to variation 
by secondary modification, it is the gill. While it is an important organ, it seems as 
if it were hardly possible to use it as a basis of classification for the whole group, as 
Pelseneer has done. In a group where any and all organs are so subject to secondary 
modification, there must be a careful Comparison of many of them, instead of one or 
two, perhaps 5 and even a complete knowledge of the comparative anatomy, which we 
by no means possess, can not be safely used as a basis for classification without the 
aid of comparative embryology, which is still less known. 
The phytogeny of the gills . — It is generally considered that the anatomy of those 
lamellibranchs which possess plate gills (Nucula, Solenomya, etc.) shows conclusively 
that the group which they form must be the most primitive one of all living lamelli- 
branchs: More especially since the appearance of Mitsukuri’s paper on these plate 
gills (ISf o. 13) has this been the general opinion. Their anatomical and histological 
similarity to certain gasteropod gills is one of the strong points for such a belief. 
But there is a very great gap between these plate gills and the strictly filamen- 
tous type, which, it seems to me, can not be explained by any facts which we now 
possess, either anatomical or embryo logical. 
In his Challenger report on the Mollusca, Pelseneer attempts to show by a series 
of diagrams the phylogenetic development of the gill. Beginning with Malletia , with 
plates extending laterally, he derives Nucula from it, in which the outer end of the 
plate is turned slightly downward. He now has to interpolate an hypothetical gill in 
which the plates have developed ventrally for some distance, but which shows no sign 
of an ascending portion. The next stage which he takes in his phylogenetic develop- 
ment is represented by the gill of Area, in which there is a fully formed ascending 
limb of the filament and neither limb shows anything plate-like, but both are cylin- 
drical. But right here is the gap referred to, and it would still remain very great 
even if his hypothetical gill just preceding it were really known to exist. 
This scheme of historical development was molded upon what little knowledge 
we have of the ontogeny of the gill. This knowledge, except for a few fragmentary 
observations, we owe to Lacaze-Duthiers (No. 7). In his study of the development of 
the gill of Mytilus , he shows that the gills appear from a ridge running horizontally 
along the side of the body, from which rods grow out and descend ventrally. These 
are separate from one another and become the filaments. After attaining to a certain 
length their enlarged ends fuse together and form a solid membrane. The lower edge 
