MORPHOLOGY OF LAMELLIBRANCHIATE MOLLUSKS. 401 



tissue layer {ct). The tianspareut, 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 frregular secondary folds on the side of the primary folds have given 

 way to one regular secondary fold («/). 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 

 lieavy fringe, part of which is indicated in the i)alp just anterior to the mouth at /r, 

 Fig. 43, PI. Lxxxv. Fig, 68, PL lxxxviii, represents a small portion to show the 

 nature of this fi-inge. The small figure at the right (A) represents a large branch, 

 whose base is continuous with the ft-ee 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-shaj)ed. Fig. 68 (B) represents a part of 

 the extreme tii) of the fi-inge more highly magnified. Here the concave side of the 

 mass is i)resented, 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 

 ( [uite a definite row in their outer third. At many points in this epithelium are found 

 groups of large gland cells {glc). At the base of the cells is a more or less definite 

 basement membrane {hm). Eunning through the compact tissue of the fi-inge are 

 blood vessels whose flat, bounding endothelium is plainly seen [hv). 



The labial palps take food collected upon the gills fi-om 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 fiinge about the 

 mouth of Pecten, though situated at the posterior end of the palji. The appendage in 

 JV»cwto, 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 cesojjhar/ns. — 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, FissureUa, HaUotis, and Dentalium. 



The (ES0i)hagus 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 



