ABERRANT FORMS OF LAMELLIBKRANCHIATE GILLS, 603 
a loose trabecular tissue with many nuclei and within whose 
network blood can flow. The space between the chitinous 
bars (A, fig. 11), which is quite large in Yoldia, seems to be 
tolerably free from this lacunar tissue. Fig. 11 @ gives the 
outline of the plate seen from one side. 
Theoretical Considerations. 
The gills, here described, of Nucula and Yoldia are, 1 
think, the most rudimentary of any that have been studied 
so far. In fact, at first sight, the resemblance to the ordi- 
nary Lamellibranch gill is not apparent, and they suggest 
more the Cephalopod gill. But I believe, the homology of 
their various parts with those of more complex gills in 
Unio, Mytilus, Arca, &c., is not difficult to make out. 
After consulting the articles by Peck (loc. cit.), Posner,! 
Lacaze-Duthiers,? Bonnet,®? and others, and also after ex- 
amining the sections I myself have obtained of Unio, 
Modiola, Scapharca, &c., 1 have no doubt whatever that the 
plates in Nucula and Yoldia represent the descending or 
attached limb of the filaments in the owter and inner gill- 
plates in forms like Mytilus, Modiola,and Arca, and accord- 
ingly are homologous with the folds on the izmer lamella of 
the outer gill-plate, and on the outer lamella of the inner 
gill-plate in Unio, Anodon, and Dreissena. If a comparisou 
is made of my-fig. 6 with any of the cross-sections of gill- 
filaments given by Peck, it will be seen at once how 
similarly the paired chitinous bars are placed, how almost 
identically the epithelial cells are arranged around them, 
how two rows of those cells (/. f,, fig. 6)—called by Peck 
latero-frontal epithelial—have longer cilia than the rest. 
In fact, Peck’s fig. 12 (a transverse section of a filament of 
the Anodon gill) agrees with my fig. 6 in all essential points. 
The left hand-figure in his fig. 5 (the superficial view of the 
edge of a gill-filament of Mytilus showing the latero-frontal 
and other epithelial cells) and the upper’part of his fig. 20 
(the same view of a gill-filament of Anodon) would pass very 
well for the corresponding part in Nucula. So far as I can 
make out from rather poor specimens, the latero-frontal cells 
in Nucula are strikingly like those represented in Peek’s 
' Carl Posner, “ Ueber den Bau der Najadenkieme,” ‘ Archiv. fiir 
mikros. Anat.,’ 1875. 
* Henri de Lacaze-Duthiers, “‘ Mémoire sur le Developpement des 
Branchies des Mollusques Acephales Lamellibranches,” ‘ Ann, d. Sci. Nat.,’ 
Ser. iv, tome v, 1856. 
3 Robert Bonnet, “Der Bau u. die Circulations-verhiiltnisse der Ace- 
phalenkieme,” ‘ Morphologisches,’ Jahrbuch iii, 1879. 
