4i8 
bulletin of the bureau oe fisheries. 
GILLS. 
There are two large gills on each side of the body which extend parallel to its longi- 
tudinal axis from the neighborhood of the mouth almost to the posterior extremity of 
the mantle. (Tig. 16, pi. xlviii.) They are attached to the body by a suspensory mem- 
brane in the usual manner. The gills are much alike except that the inner one of each 
pair is somewhat broader than the outer and hence reaches below its fellow. They are 
pointed at the extremities and anteriorly are inclosed by the palps. Each gill consists 
of two lamellae which lie close together; or perhaps it is more correct to think of it as 
being composed of a single lamella which has been folded upon itself. According to this 
conception, the gill consists of a direct and a reflexed lamella, the two being continuous 
at the free edge of the gill. Various anatomical and embryological studies, especially 
those of Eacaze-Duthiers (8) and Peck (12), show this to be the correct interpretation. 
The two lamellae are united to each other merely by blood vessels which pass from the one 
to the other. The interlamellar space is not partitioned off into definite parallel water 
tubes by continuous septa, but is undivided except for the scattered blood vessels which 
traverse it. The only place where there is anything resembling true interlamellar septa 
is at the upper borders of the gills where nonvascular, or only partially vascular, strands 
bind the two lamellae together. At the outer free edge of the gill they are bound firmly 
together by lacunar connective tissue and by a continuous plate or cord of muscle 
whose fibers run longitudinal to rather than transverse to the gill. By its contraction 
the gill is shortened and folded. A large nerve lies immediately above this muscle, but 
I have made no attempt to study its distribution. 
Each gill is attached to the suspensory membrane by one lamella only, as is common 
among lamellibranches. The inner lamella of the outer gill and the outer lamella of the 
inner gill are attached to the suspensory membrane as far back as the adductor muscle. 
From this point they are attached to each other. The outer lamella of the outer gill 
is attached to a ridge on the mantle (the same as that mentioned above). The inner 
lamellae of the inner gills of the two sides are united to each other, except at the extreme 
anterior, where for a space of half an inch or so they are attached to the sides of the 
byssal apparatus which with the foot extends ventrally at this point. The attachments 
of the gills, together with the intermantle septum, thus cut off a system of supra-bran- 
chial passages from the general mantle cavity. A section across the body shows that 
there are three of these, which are diagrammatically represented in figure 2 (r, 2 , and 3 ). 
A section taken posterior to the visceral mass would show only a single suprabranchial 
passage, the three having been thrown together at the termination of the suspensory 
membranes. This single suprabranchial passage lies below and posterior to the adduc- 
tor muscle, and for distinction might be called the cloacal chamber. 
The direct lamellae are outgrowths from the suspensory membrane, and hence 
there is a firm organic union between them. On the other hand, the attachment of the 
upper borders of the refiexed lamellae to the neighboring parts and of the ridges on the 
mantle to each other, to form the intermantle septum, is merely an interlocking of cilia 
so that they can be torn apart without doing the slightest injury to the animal. Only 
