MORPHOLOGY OF LAMELLIBRAN CHI ATE MOLLUSKS. 
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straining-line cells. The exterior epithelium is of long, large cells in the outer half 
of the filament, gradually merging into short cells, which are considerably widened. 
At the inner edge of the filament they again become columnar, but are not so long as 
at the outer edge. 
Gland cells (glc) are present at the lateral boundaries of the frontal region (/) 
and are very conspicuous in certain specimens. They always occupy this position, 
and I have, never seen them in the middle of the frontal epithelium, and only occa- 
sionally hr other parts of the filament. These mucous cells vary in shape in different 
forms, sometimes being spherical, sometimes long and slender. They may be close to 
the base of the epithelium, surrounding them or pouring their contents out upon its 
surface. These various shapes and positions, more or less common in all lamelli- 
branch gills, are shown in Fig. 85, a representation of the inner edges of four filaments 
of the gill of Anomia. 
The cells of the epithelium immediately surrounding the gland cells in Pecten are 
seen to be crowded with yellow pigment granules (pg 1 Fig. 83). 
At some little distance inward from the gland cells, appear in section four or five 
cells, bearing long cilia, crowding together to form the single compound straining line 
(i/7) on either side. These lines of cilia are inclined outward. 
Fine cilia exist upon the frontal surface (/), becoming gradually shorter and 
finally disappearing as they reach the region of the gland cells. It is the movement 
of these cilia which causes the currents of water over the surface of the gills. 
The chitinous layers, between which is the blood space of the filament, are lined 
on their inner surfaces by a distinct endothelium. Both the nuclei and protoplasm of 
its cells may be plainly seen ( en ). The blood channel, in which appear numerous 
nucleated blood corpuscles (fee), is divided by a transverse partition. Pelseneer 
describes this septum in Pecten opercularin as being chitinous in nature (though he 
does not use that term). I believe this septum (p) to be made of cells continuous 
with the endothelium of the walls of the blood cavity. Nuclei are very frequently 
present in it, often larger than the one represented in Fig. 83. 
In the form referred to, Pelseneer also describes two large cells, one on either side 
of the frontal region — the latero-frontal cells of Peck-r-with their long cilia. Nothing 
of the kind exists in Pecten irradians. 
The filament of the reentering angle (Fig. 86, ra) is much enlarged. Its filamen- 
tous nature is recognized by the chitinous layers (eh), the compound straining line of 
ciliated cells (Ifl), and the blood channel (be). The epithelium of its walls is of a nearly 
uniform appearance. The descending and ascending limbs are connected throughout 
their entire length by a continuation of their walls ( w ), which thus separate the interior 
space of the gill between the lamellae into a single channel for every fold. The walls 
of the partition are generally applied to one another in sections, but very frequently 
spaces are found between them which contain blood corpuscles. I believe that a vas- 
cular connection exists here between the two gill lamellae, though it is said by Pelse- 
neer not to exist in P. opereularis. Lacunar tissue also seems to be present (lac). 
The ciliated junctions are peculiar and, so far as I know, are found only in Pec- 
ten. Mytilus, Area (Fig. 66), etc., possess filaments joined to each other by ciliated 
discs, situated on the sides of the filaments. In Pecten , however, the ciliated junc- 
tions do not occur upon the sides of filaments, but only upon conical projections from 
their inner surfaces. These are shown in Fig. 84, a diagrammatic representation of 
