1 62 BULLETIN OF THE BUREAU OF FISHERIES. 



in contact with the object to which the thread is to be attached. According to Wil- 

 liamson (1907), the secretions pour out from the white gland so that they surround the 

 stem and fill up the groove, the flow probably being caused by internal pressure attained 

 by distending with fluid the lacunae which exist between the muscles and about the 

 glands. In the depression at the end of the foot ducts from the purple gland pour out 

 a cementlike secretion which forms the attachment plate for the thread. The secretions 

 are thick when first discharged and of fibrous character. As soon as they are in place 

 the lips of the groove open and allow the sea water to enter, which hardens them. This 

 results in the formation of a thread which at the proximal end loops the byssus stem 

 and at the distal end is cemented to some solid object by means of the attachment plate. 

 In color the new threads are a glistening white, but in a few hours' time they become 

 yellowish, then brownish, and when old may be of a very dark-brown shade. 



The rate at which the threads may be formed was determined with some specimens 

 about 1 inch long and which were probably less than 1 year old. It is advantageous to 

 use young mussels for these observations, because after being transferred to a dish of 

 sea water they become active far more quickly than do older shellfish and the pro- 

 duction of byssal threads begins within a few minutes after they have crept about. 



a ° c d e f 



146 



Fig. 146. — Tracing of a path of byssus threads left behind by a mussel 1 inch long during a period of three days. 



In December, when the sea water was at a temperature of 45 ° F., two mussels, 

 each 1 inch long, were placed in a 2-gallon battery jar containing sea water to a depth 

 of 6 inches. In less than five minutes the shellfish began to creep about. When they 

 came in contact with the side of the jar they promptly started to ascend the perpendicular 

 wall and continued upward until they reached the surface of the water. There they 

 stopped and began to attach themselves by means of byssal threads. The foot was 

 extended and the cuplike depression at the end of the groove was pressed against the 

 glass. The cavity of the depression appeared to become filled with a white cushion- 

 like substance which flattened against the glass. With a strong hand lens the author 

 tried to see what was taking place at this point of contact but could detect nothing. 

 When, however, the foot was removed at the end of eight minutes a thread was found 

 attached in this position by the usual attachment plate. The process was immediately 

 repeated, this time the thread being formed and attached in three minutes. Then a 

 third one was formed during the following five minutes. Four hours later this same 

 mussel was found attached by 18 threads. 



In another jar 14 young mussels were placed and left for a period of 2,% hours, when 

 4 of them were found attached to the side of the jar near the surface of the water with 

 10, 13, 15, and 16 threads, respectively. 



