Jan., 1903.] Development of the Gill in Mytilus. 
51 
PRELIMINARY REPORT ON THE DEVELOPMENT 
OF THE GILL IN MYTILUS. 
Edward L. Rice. 
(Abstract.) 
The earl}’ development of the gill of this form was very thor- 
oughly worked out by Lacaze-Duthiers in 1856. To his account 
of the development of the earlier filaments the present writer has 
nothing to add. As described, a papilla is formed, growing down- 
ward from the gill axis, and is reflexed on it.self, giving rise to 
the familiar U-shaped filament. Later filaments follow a very 
different scheme, there being no such bending of an originally 
simple filament. At the posterior end of the curiously bent gill 
axis a series of thin transverse ridges are developed. At first the 
edge of each ridge is entire ; but growth is very early checked in 
the center, so that the ridge is divided into two flat, rounded 
lobes, corresponding respectively to a filament of the outer and 
one of the inner gill plate. As the lobe elongates it becomes per- 
forated at its proximal end, thus being resolved into the two 
branches of a U-.shaped filament identical in form with those first 
developed. This mode of development of the later filaments has 
been observed in Modiola, Area, Anomia, and Mya ; the earlier 
filaments have been studied only in Mya, where they correspond 
with Mytilus. 
An interesting parallel is seen in the development of the inter- 
lamellar connections. The interlamellar connection, in its finished 
form, is a simple bar, containing a blood channel, and connecting 
the two branches of one and the same filament. In an early stage 
the two branches are connected by a continuous plate of tissue 
extending from the bend of the filament upward for a short dis- 
tance. This stage is exactly comparable with the adult conditions 
in -\rca and Modiola. Later a perforation appears in the plate, 
and the portion above the perforation is transformed into the 
characteristic bar-like connection. 
.\s yet the examination of sections is too little advanced to allow 
any detailed statement concerning the mode of perforation in 
either case. 
Ohio Weslevaii University. 
