OF MASSACHUSETTS. 51 



be transferred in this way from one locality to another. Shallow flats 

 covered with thick eel grass are usually the most productive of heavy 

 " sets," although the exposed nature of these flats during the winter 

 often causes a severe mortality among the young scallops. 



The exact conditions governing the set in any one locality are diffi- 

 cult to observe. The primary requisite is something to which the attach- 

 ment can be made. This is usually eel grass. In a number of cases 

 heavy " sets " are found in the still water on the sides of a swift 

 current. This is often the case at the entrance to harbors where eel- 

 grass flats Mne the channel. The spread of the incoming or outgoing 

 waters carries with it the young larvse, which, striking the eel grass in 

 the stiU water, settle upon the waving blades. 



The Bt/ssus (Fig. 43).- — The young scallop after its free swimming 

 existence attaches itself by slender strands of hard, gelatinous material 

 to the first suitable object with which it comes in contact. This bundle 

 of threads is called the byssus, and is similar in functicfn to the anchor- 

 ing strands, the "beard" or "weed," of the common black mussel. 

 The number of fibers composing the byssus depends upon the size of 

 the scallop and the length of time attached, as but one thread is formed 

 at a time, and the total number is not at once completed. As the scallop 

 iucreases in size, the number of strands increase in proportion to the 

 added weight. The environment may also determine the strength of 

 the byssus, as scallops exposed to the strong winds and wave action 

 necessarily need more anchoring strands. 



The byssal threads pass from a gland in the foot out through an 

 indentation in the lower or right valve of the scallop to the surface of 

 the foreign object to which they are attached by minute discs. This 

 indentation, directly under the anterior " ear," is the so-called byssal 

 notch, which has already been described in chapter III. Along this 

 groove are little projecting teeth or knobs, which develop in the later 

 part of the dissoconch stage soon after the attainment of the byssal 

 attachment. The use of these teeth is not known, but appears to be 

 related to the byssal habit. Possibly they are of use in separating the 

 strands. In scallops under one year of age these teeth number four to 

 five, but in the majority of old specimens they are entirely absent, 

 evidently disappearing when the byssus becomes practically useless, 

 as the last formed teeth are rounded instead of sharply pointed. The 

 manner of disappearance is readily shown by breaking the valve along 

 the byssal groove and observing the line of teeth which have been en- 

 veloped in the adult shell. As they are formed at the same time that 

 the byssus becomes functional, and disappear when that organ is no 

 longer of use, there seems little doubt that their use is closely correlated 

 with that of the byssus. 



The following excellent description of the process of byssal fixation 

 is given by Jackson (4) : — 



