MORPHOLOGY OF LAMELLIBKANCHIATK MOLLUSKS. 421 



straiuing-liue cells. The exterior epitheliniii is of long', lai.ii'e cells in the outer half 

 of the filanieiit, onnlually mergiuj; into short cells, which are considerably widened. 

 At the inner edge of the lilament they again become colnnuiai'. but are not so long as 

 at the outer edge. 



Gland cells {(jlc) 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 m other parts of the filament. These mucous cells vary in shape in different 

 forms, sometimes being si)herical, sometimes long and slender. They may be close to 

 the l)ase of tlie epithelium, surrounding them or pouring their contents out ui>on its 

 surface. These various shapes and positions, more or less common m all lamelli- 

 l)ranch gills, are shown in Fig. 85, a re])resentation (»f tlic inner edges of four filaments 

 of the gill of Aiioniia. 



The cells of the epithelium immediately siirrcmnding the gland cells in Peeten are 

 seen to be crowded with yellow ])igment granules {pg. Fig. <S3). 



At some little distance inward tiom the gland cells, ai)pear in section four or five 

 cells, bearing long cilia, crowding together to form the single coinixamd straining line 

 (///) on either side. These lines of cilia are inclined outward. 



Fine cilia exist upon the frontal surface (/), becoming gradually shorter and 

 JBlnally disappearing as they reach the region of the gland cells. It is the movement 

 (»f 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 i)lainly seen {en). The blood channel, in which ai)pear numerous 

 nucleated blood corpuscles (6c), is divided by a transverse partition. Pelseneer 

 describes this septum in Peeten operciUaris 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 fr-equently 

 present in it, often larger than the one represented in Fig. So. 



In the form referred to, Pelseneer also describes two large cells, one on either side 

 of the fi"ontal region — the latero-fiontal cells of Peck — with their long cilia. Nothing 

 of the kind exists in Peeten irradians. 



The filament of the reentering angle (Fig. 86, ra) is much enlarged. Its filamen- 

 tous nature is recognized by the chitinous layers (c/^), the compound straining line of 

 ciliated cells (^V), 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 theii- walls (w), which thus separate the interior 

 space of the gill between the lamellae into a single channel Ibr every fold. The walls 

 of the partition are generally applied to one another in sections, but very freipiently 

 spaces are found between them which contain blood corpuscles. I believe that a vas- 

 cular connection exists here between the two gill lamelhe, thojigh it is said by Pelse- 

 neer not to exist in P. operenlaris. Lacunar tissue also seems to be present {Jae). 



The ciliated junctions are peculiar and, so far as I know, are found only in Pec- 

 ten. Mytilvs, Area (Fig. 66), etc., ]iossess filaments Joined to each other by ciliated 

 discs, situated on the sides of the filaments. In Peeten, how«'ver, 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 



