SEA MUSSEL MYTILUS EDULIS. l8l 



Round, pigmented spots found on the lips of some mussels are similar in character 

 to the pigmented cells of the mantle edge, except that the cells are taller and more irreg- 

 ular in form. In size the cells vary from 38 to 40 microns in length by from 2 to 3 mi- 

 crons in width. The pigment granules are finer than those of the cells on the mantle 

 edge, are more abundant, and extend into the basal regions of the cells. At the distal 

 ends of the cells the granular group tapers out to a point (fig. 170, p. 180). Each cell 

 bears many cilia that arise from a layer of basal bodies which stain deeply with haema- 

 toxylin. The cilia are about 7 microns in length. The nuclei which lie at the proximal 

 ends of the cells are long and narrow and filled with fine chromatin granules. The por- 

 tion of the cells distal to the nuclei is filled with finely granulated, yellowish-brown 

 pigment. Large gland cells more or less oval or elliptical in outline occur at frequent 

 intervals, between the proximal ends of the pigmented cells (fig. 170, GIC, p. 180). 



PHYSIOLOGY. 



The functions of the various sense organs just described in Mytilus remain undeter- 

 mined up to the present time. Literature on the subject is of a speculative nature, but a 

 brief review of it may be worth while. 



In regard to the sensory cells scattered throughout the epidermal layer, Dakin (1909) 

 says they seem to be stimulated by very slight movements in the water. He also ventures 

 the assumption that in addition to being tactile organs they may be olfactory in function. 



Concerning the function of the osphradium there is no experimental evidence. Be- 

 cause of its similarity to the osphradium of gastropods such authors as Lankester (1883) 

 and Pelseneer (1888) have assumed that it is olfactory in function. 



The abdominal sense organ, according to Dakin (1909), functions to test the quality 

 of the incoming water either as an olfactory or as a gustatory organ; but since this sense 

 is usually ascribed to the osphradium, it is difficult to understand why these two organs 

 should be placed side by side. Since the histological structure is so remarkably like 

 that of the lateral line organs described by Eisig (1887) in the Capitellidae, Thiele 

 (1889 and 1890) assumes that they are homologous with the lateral line not only of the 

 chaetopods but of fishes, and therefore probably function to perceive wave movement or 

 vibrations in the water. He considers furthermore that they may be olfactory in func- 

 tion. 



No literature on the function of the otocyst in Mytilus has been found, but it may be 

 assumed that, as in other invertebrates, it serves as an organ of orientation. 



Neither has the author been able to find any references to the physiology of the 

 eyes or pigmented epithelium of the mussel. A single series of experiments performed 

 September 6, 1912, furnishes all the data the author has. Some mussels, whose mantle 

 edges were expanded from between the open valves, were lying in a trough of running 

 water in the laboratory, so situated that a number of them were subjected to the direct 

 light of the sun while the others were shaded. When the hand was held so as to cast a 

 shadow on the mantle fringe a rather quick response followed. The mantle edge con- 

 tracted decidedly and sometimes was completely withdrawn, followed by closing of the 

 valves. A similar response was obtained from the mussels lying in an expanded condi- 

 tion in the shade when direct sunlight was reflected with a small mirror onto the mantle 

 edge. Response came, however, only when there was a decided change in the intensity 

 of the light one way or the other. This would indicate, therefore, that the pigmented 

 epithelium of the mantle edge is a light receptor, but at best is a very crude sense organ. 



