MORPHOLOGY OF LAMELLIBRANCH IATE MOLLUSKS. 
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tissue layer (ct). The transparent, irregular cells composing the main body of the 
palp, and extending into the folds, are the same as those found in the mouth and 
walls of the visceral mass of Ostrea, and described by Prof. Brooks (No. 3) as fat cells. 
Fig. 63 is taken from about the middle of the palp. Here the folds are more 
narrow, and the irregular secondary folds on the side of the primary folds have given 
way to one regular secondary fold (sf). In Fig. 64, a section close to the attached 
border of the palp, the folds are very regular in size, and without secondary ridges. 
The palps of Pecten are peculiar in having upon their free edges near the mouth 
a number of projections which are extremely convoluted and give the appearance of a 
heavy fringe, part of which is indicated in the palp just anterior to the mouth at fr, 
Fig. 43, PL lxxxv. Fig. 68, PI. lxxxviii, represents a small portion to show the 
nature of this fringe. The small figure at the right (A) represents a large branch, 
whose base is continuous with the free edge of the palp near the mouth. The trunk of 
this tree-like mass is not solid, but is merely a thin sheet of palp tissue whose surface 
nearest the mouth is concave. The outer surface shown in the figure is convex. A 
section across it would be somewhat crescent-shaped. Fig. 68 (B) represents a part of 
the extreme tip of the fringe more highly magnified. Here the concave side of the 
mass is presented, and it shows that the whole fringe is made by flat, sheet-like out- 
growths of the edge of the palp. The edges of these flat projections turn inward 
toward the mouth. These edges seem to have grown a great deal more rapidly than 
the interior of the sheet, and have thus been forced to convolute themselves greatly, 
as represented in the figure. 
A section through the ends of this fringe shows an epithelium made up of very 
much elongated, ciliated cells (Fig. 75, PI. xc, ep), whose nuclei are arranged in 
quite a definite row in their outer third. At many points in this epithelium are found 
groups of large gland cells (glo). At the base of the cells is a more or less definite 
basement membrane (6m). Running through the compact tissue of the fringe are 
blood vessels whose flat, bounding endothelium is plainly seen (bv). 
The labial palps take food collected upon the gills from the anterior ends of the 
latter, and by the cilia pass it on into the mouth. The movement of food particles 
here does not seem so rapid as upon the gills, and its path seems much less defined 
than the latter. This is easily seen upon the palps of Yoldia. The long, ciliated palp 
appendage of this form with its convoluted borders is similar to the fringe about the 
mouth of Pecten , though situated at the posterior end of the palp. The appendage in 
JYucula, like that of Yoldia , is supposed by Mitsukuri (No. 13) to serve in collecting food, 
and the mouth fringe of Pecten in all probability has a similar function, the products of 
its gland cells cementing the food particles together as on the gills, and its ciliated 
epithelium passing this on down its concave inner surfaces to the mouth. 
The oesophagus. — In the Nuculidce , Pelseneer (No. 17) has described in the buccal 
region of the digestive tract a transversely enlarged glandular portion, called the 
pharyngeal cavity. It occurs in no other lamellibranch, but he thinks is homologous 
with a cavity found in other mollusks — Patella , Fissurella, Haliotis, and Dentalium. 
The oesophagus proceeds nearly vertically upward to the stomach. It is very large 
in Pecten (Fig. 43, mo), smaller in Mytilus (Fig. 33) and Venus (Fig. 10). Its opening 
into the stomach is generally somewhat funnel-shaped, but in Mya is abrupt, with a 
definite muscular opening. 
F. C. B. 1890—26 
