SEA MUSSEL MYTILUS EDULIS. 1 63 



Hescheler, in Lang (1896), states that most of the byssus-forming mussels are able to 

 cast off the byssus and again replace it with a new one and many forms are able by alter- 

 nately fastening threads forward and breaking off the old threads behind to move up a 

 smooth, perpendicular glass wall. The sea mussel not only uses this method of movement 

 on perpendicular glass surfaces but on horizontal surfaces covered with mud, as William- 

 son (1907) has described. Figure 146 represents a trail of byssus threads left by a mussel 

 which was under the author's observation in the laboratory. It was in a glass dish half 

 filled with sea water. The animal at first crept up the vertical side of the dish to the sur- 

 face of the water, where it attached itself at a, then it started to move around the wall of 

 the dish in the direction indicated by the letters b, c, d, etc., keeping just below the sur- 

 face of the water. The distance of 7^2 inches was covered in three days, at the end of 

 which period the mollusk attached itself permanently with 18 threads and remained 

 there until it died 10 days later, probably from starvation. 



CHEMISTRY OF BYSSUS. 



The composition of byssus is similar to that of the organic matter which is present 

 in the shell. It is popularly spoken of as a horny or chitinous material, but in the opinion 

 of Krukenberg (1886) it is closely allied to conchiolin, which forms the organic basis of 

 the shell. According to Abderhalden (1908), it yields on hydrolysis glycocoll, tryosin, 

 and proline in large amounts, besides alanine and aspartic acid. Treated with nitric 

 acid the threads are stained yellow, which is a typical protein reaction. These results 

 clearly indicate that byssus belongs in the albuminoid group and not in the class of chitin. 



In regard to solubility .Winterstein (1910) states that the byssus threads are insolu- 

 ble in boiling water, alcohol, ether, ammonia, dilute acids, or alkalis, but are slightly 

 soluble in hot concentrated acetic acid or in concentrated mineral acids. The solution in 

 acetic acid is precipitated by tannin or by mercuric chloride. According to Scharling 

 (1842) and Schlossberger (1856), byssus is slightly soluble in potassium hydroxide. 

 Byssus is classified by Hammarsten (19 14) as a skeletin compound in the group of albu- 

 minoids. 



RESPIRATORY SYSTEM. 



ANATOMY. 



The organs of respiration in the Mollusca are usually the gills, but in the mussel, as 

 in other lamellibranchs, the primary function of these organs seems to be the collection 

 of food. The respiratory function is carried on only in part by the branchial apparatus. 

 Aside from the gills, respiration takes place through the plaited membranes which extend 

 across from the base of the gills to the mantle. To these folds Sabatier (1874) gave the 

 name Organes godronncs. Respiratory exchange also takes place through the surface of 

 the body, especially on the inner wall of the mantle. 



The gills are suspended from the ventral side of the visceral mass on either side of 

 the body (fig. 147, Br, opp. p. 163) and extend from the corners of the mouth to the bran- 

 chial membrane (fig. 120, Br, p. 141). The posterior portion which extends from the ven- 

 tral surface of the posterior adductor muscle to the branchial membrane is not attached 

 to the body, but is supported by the large afferent branchial blood vessel which runs for- 

 ward on its dorsal edge (fig. 135, ABV, p. 153). The right and left gills are most widely 

 separated from each other in the middle of the body and from this point converge toward 

 each other both anteriorly and posteriorly. 



