606 K. MITSUKURI. 
gills arise first as.a ridge on each side of the body. Leydig? 
makes the same statement. M. Lovén’s® observations have 
a still more important bearing on the point. He says :— 
‘* Nous avons, si je ne me trompe, vu la premiére formation 
des branchies ; nous en savons assez pour étre sir qu’elles 
se montrent sous la forme d’un cordon fin, renflé a certains 
intervalles ; que ces renflements se contournent plus tard en 
anses, qui s’allongent de plus en plus, et sur lesquelles se 
développent les cils vibratiles reguliérement disposés et d’un 
forme particuliére.”? “Un cordon fin renflé a certains in- 
tervalles” is, it seems to me, nothing but a ridge with slight 
swellings or papille. Lovén’s figures are not exactly clear 
to me, but what he designates as the gills are certainly in 
favour of my view. In all the fragmentary embryological 
observations, the gills are generally seen as papill, or 
nothing but the folds of a blood-channel. I have already 
called attention to the anterior part of the Yoldia gill where 
the plates die out and the gill is continued simply as a 
ridge containing a blood-channel. Whether this is a rem- 
nant of the primitive ridge or not it is difficult to determine, 
but the fact that there can be on the side of the body a thin- 
walled ridge which, containing a blood-channel, must serve 
more or less for respiration, goes far in support of the view 
here advanced. 
To review the whole matter, the Lamellibranch gill was 
perhaps originally a simple ridge on the side of the body, 
but to increase the surface of contact with the water folds 
may have arisen on two sides of this ridge. If such was 
the case, Nucula and Yoldia are still in a stage only very 
little advanced from this primitive condition. In course of 
time, however, as some of the Lamellibranchiata, either 
owing to degeneration or some other cause, become incapable 
of extensive locomotion, these buds or folds were perhaps 
prolonged to form tentacular filaments, which, going on in 
their development, finally produced such complex gill struc- 
ture as we see in Mytilus, Unio, Ostrea, and other forms, 
taking on at the same time functions totally foreign to their 
1 Franz Leydig, “ Ueber Cyclas cornea,” ‘ Miiller’s Archiv,’ 1855. He 
says :—“ Die letzte Hauptmandering im dusseren Habitus esfahrt der 
Embryo durch die Bildung der Kiemen. Auch sie wachsen als Leisten von 
hinten nach vorne und zwar geben sie urspriinglich yon Mantel aus ” 
. 62). 
Bidnae till Kamedornen om utvecklingen af mollusca acephala 
Lamellibranchiata,” ‘Memoirs of the Academy of Stockholm, 1848, 
lately reprinted in an abridged form in German. ~ 
§ Translated by M. Young, and quoted by Lacaze-Duthiers in the article 
already referred to. 
