604 K. MITSUKURI. 
fig. 20. If, then, the plates in the gills of Nucula and 
Yoldia represent the gill-filaments in other genera, it 
follows from the embryological observations of Lacaze- 
Duthiers’ (loc. cit.), and from the position of the chitinous 
bars in the plates, that they are homologous with the de- 
scending limb of the gill-filaments in ordinary Lamelli- 
branchs. Professor Huxley seems to have no doubt what- 
ever of the homology stated here, as will appear from the 
quotation given further on. Admitting, then, that this sup- 
position is correct, and that the gills in Nucula and Yoldia 
are in an unusually rudimentary condition, what light, if 
any, do they throw on the organogeny of the Lamellibran- 
chiate gill? But, before proceeding to the discussion of this 
point, let us review briefly what theories have been ad- 
vanced as to what is the most primitive type of the bran- 
chie of this group. Setting aside older authors like 
Williams and Hancock, I consider the articles, already 
_ alluded to, by Peck, Posner, and Lacaze-Duthiers as having 
the most important bearing on the subject. Posner, after 
a careful histological examination of the gills of Anodon, 
Unio, Cardium, Mya, Mytilus, Ostrea, Pecten, Pholas, 
Pinna, Scrobicularia, Solen, Solecurtus, and Venus, puts 
forward, although with hesitation, the theory that the pouch- 
like gills of the Unionide are the most primitive type of the 
Lamellibranchiate gill. Stepanoff,! so far as I can gather, 
inclines to this view. Peck, on the other hand, after an 
investigation of Arca, Mytilus, Anodon, and Dreissena, 
comes to the conclusion that “the gill-plates of the Unio~ 
nide are a highly modified form derived from a simple con- 
dition in which the gills consist no¢ of plates but of a series 
of juxtaposed independent filaments, such as we see in a less 
modified state in Arca and Mytilus.” This view is the 
more generally accepted of the two. The only complete 
history of the development of the Lamellibranchiate gill by 
Lacaze-Duthiers (loc. cit.) and all the fragmentary embryo- 
logical observations on the organ show that the gills are at 
first of a tentacular or filamentary character. Those who 
read carefully Mr. Peck’s paper, will, I think, feel con- 
vinced by the arguments he brings forward. So high an 
authority as Professor Huxley is entirely of this view. He 
says :—“In its simplest form, the branchia of a Lamelli- 
branch consists of a stem fringed by a double series of fila- 
ments (e.g. Nucula). The next degree of complication 
arises from these filaments becoming, as it were, doubled 
* Paul Stepanoff, ‘ Ueber die Geschlectsorgane und die Entwicklung 
von Cyclas,” ‘ Archiy f. Naturgesch,,’ 1865. 
