SEA MUSSEL MYTILUS EDULIS. 177 



from view between the pair of labial palps. It is most easily made accessible for obser- 

 vation by laying the two gills apart and following up the inner gill support to its ante- 

 rior end, where a little dark spot which represents the eye may be easily observed, especi- 

 ally if use is made of an ordinary hand lens. It is an invaginated cup, oval in form and . 

 consisting of epithelial cells filled with coarse granules of a dark-brown pigment (fig. 

 1 66). 



In the larval mussel the eye occupies this same position at the base of the inner 

 first anterior gill filament. In young specimens that have been fixed in toto the pair 

 of eyes stand out distinctly under the microscope as large, pigmented oval spots. 



In addition to this pair of complex direction eyes, the sea mussel has a wide area 

 of its body, extending the whole length of the free mantle edge and over the entire sur- 

 face of the anal membrane and the foot, covered with brown densely pigmented epi- 

 thelial cells, which are capable of being stimulated by light. Sometimes spots of the 

 pigmented epithelium are present on the lips, although their presence in this position is 

 unusual. 



HISTOLOGY. 



The pallial sense organs, which are most commonly found on the ventral epi- 

 thelium of the posterior adductor muscle, consist of small groups of cells that are so 

 related as to form little sense hills (fig. 164, p. 178). The surrounding epithelium con- 

 sists of cubical cells covered with a relatively thick cuticula and bearing no cilia. The 

 outer cells of the pallial sense organs are slightly taller than the epithelial cells, and as 

 one passes from the periphery to the center of the sense body the cells become taller, 

 the central cells being about twice as high as those of the surrounding epithelium. They 

 are furthermore characterized by being ciliated, the cilia of one side of the elevation 

 being much longer than those of the opposite side. The longest cilia are about as long 

 as the tallest cells. The nuclei are large and contain several large chromatin granules, 

 and each nucleus gives off from its base a nerve fiber. These fibers pass down into the 

 connective tissue below, where they unite into a trunk that apparently runs to the 

 visceral ganglion. 



The pinselzellen of Flemming (1870), which are scattered over the epidermal layer 

 of the mantle cavity, differ but little in form and size from the epithelial cells which 

 surround them. Sometimes they are narrower or their outer ends flare outward. They 

 are tall, columnar cells with a height about three times their breadth. They contain 

 large oval or elliptical nuclei in which are several chromatin granules. Their most 

 characteristic feature is the group of long cilia which extends from the outer end of the 

 nucleus to a distance beyond the surface cuticula equal to the total length of the cell. 

 (See fig. 167, p. 178.) These cilia stain deeply with iron hematoxylin. Dakin (1909) in 

 his studies on Pecten found these same epidermal sense cells and states that each one is 

 connected with a nerve fiber. The author has failed to demonstrate any nerve connection 

 in Mytilus. 



The osphradium is conspicuous because of its large columnar cells and nuclei which 

 stand out in sharp contrast to the small cubical epithelial cells that surround it. The 

 organ extends on either side of the body from a point ventral to the visceral ganglion 

 to the inner side of the gill support and is one layer of cells thick. (See fig. 172, p. 180.) 

 Numerous fine nerve fibers arise from the basal portion of the organ and run into the 



