SEA MUSSEL MYTILUS EDULIS. l6l 



from i to 4 mm. in length, were found attached in a mass about the upper edge of the 

 jar. These same mussels, with other young ones of larger size, were placed in a glass dish 

 containing sea water and kept under observation. In less than a minute most of them 

 thrust out the foot and began to creep about. The foot was extended for a distance 

 beyond its base nearly equal to the length of the animal itself. The tip of the foot was 

 then attached by means of its sucker, and then by contracting its longitudinal muscles 

 the body was drawn almost up to the point of attachment. Mussels i ]A mm. long ex- 

 tended the foot for a distance of 2 mm., those 5 mm. in length extended it 4 mm., while 

 mussels 10 mm. in length thrust ifout for a distance of 8 mm. before contracting it. 



The young shellfish were able to creep up the perpendicular walls of the glass dish 

 almost as well as they could over the bottom, but their powers for moving under difficult 

 conditions did not reach their limit here. A number of individuals, having reached the 

 surface of the water, continued on by creeping out on the underside of the superficial 

 film similar to the habit of the pond snail. Some of them succeeded in traveling across 

 the dish without falling, while others lost their hold and sank slowly to the bottom with 

 their feet fully extended and moving about as if in search of an object on which to anchor 

 themselves. The slow rate at which they sank suggested that they were possibly being 

 supported by ciliary action on the foot. 



Ascending a perpendicular wall and creeping on the superficial film of the water is 

 accomplished by using the entire ventral groove as a sucker. By distending and con- 

 tracting the foot and alternately attaching the posterior and anterior ends progression is 

 accomplished. Sometimes the young shellfish would allow themselves to slide down 

 the wall of the glass dish on the bottom of the foot, catching hold every few millimeters 

 in the descent and then relaxing their hold again. 



The rate of locomotion was determined by allowing the mollusk to creep over measured 

 distances which varied from 1 to 10 cm. in extent. Specimens 4 to 5 mm. in length were 

 used, and these covered the distance at rates varying from 1% to 2% cm. per minute. 

 The usual and average rate was 2 cm. per minute. 



Before describing the functions involved in the formation of the byssus it will be 

 well to state briefly the views which have been held in regard to its origin and nature. 

 Von Nathusius-Konigsborn (1877), Reichel (1888), and others took the view that the 

 byssus grew from the animal's body, as does the cuticula of arthropods, and in like 

 manner was shed from time to time. On the other hand, M tiller (1837), Tullberg (1882), 

 Jobert (1882), and Williamson (1907) have disproved the contentions of these writers 

 by clearly demonstrating that the byssus arises as a glandular product. A few observa- 

 tions of the byssus-forming habit of the mussel are sufficient to convince one that this 

 substance is a product of glandular secretion and not of cuticular growth. 



The byssus stem, which represents the fused secretions from the cells of the epithe- 

 lial walls of the glandular cells lying in the surrounding tissue, is molded into its charac- 

 teristic form first by the cavity and then by the neck leading to the opening from which 

 it passes out. Growth is continuous, but its rate probably depends upon the amount 

 of strains the byssus has to bear, vigorous stimuli causing a more rapid secretion of 

 material. Such growth is capable of producing a stem of cumbersome length, but the 

 shellfish is able to avoid this by casting it off and starting a new one. 



The threads are formed in the basal canal of the foot. When the mussel is in the 

 act of producing a new thread the foot is extended and the depression near its tip placed 



