160 BULLETIN OF THE BUREAU OF FISHERIES. 



The origin of this very specialized type of mollusean foot which is found in Mytilus 

 has been traced from the simple foot of Solenomya, which has a flat sole with a simple 

 invagination but possesses neither groove nor byssus. Nucula and Leda have this same 

 type of foot, but in addition there arises from the simple invagination a small lamella 

 and a byssus is developed to a slight extent. The next step leads to the condition found 

 in Mytilus where the invagination is differentiated into a cavity with a duct and the 

 byssus with its glands is highly developed. Being no longer primarily an organ of locor 

 motion, the foot has degenerated in size to a strap-shaped appendage without any sole. 

 Its power of extension, however, is increased to serve the chief function of attaching the 

 byssus. Parallel with this change from the primitive foot to the byssus-forming foot 

 there is a modification of the pedal muscles which become attached to the byssus gland 

 forming the retractors of the byssus. 



The byssus is a bundle of tough threads secreted by the glands lying in the foot and 

 byssus pit with which the animal anchors itself to convenient objects. It consists of a 

 great number of very thin sheets or septa of byssal matter lying between the lamellse 

 which hang down into the byssus cavity. As the byssus septa grow downward thev are 

 molded together in the form of the cavity and pass outward through the external opening 

 in the form of a solid rod, the so-called byssus root (fig. 117, ByR, opp. p. 138). Exter- 

 nally the root becomes a region of origin for numerous byssal threads which terminate 

 in specialized endings, composed of a cementlike material capable of attaching them to 

 solid objects with great firmness. 



The byssus material is light to dark brown in color and appears to be made of 

 numerous layers, one above the other, but when crushed or torn it breaks up readily 

 into fine fibrils. Tullberg (1882) pointed out that the surface layer of the thread stained 

 with carmine while the central portion did not and concluded therefore that it was a 

 different substance. Williamson (1907), however, believes that the thread is homoge- 

 neous in character and that the reaction is due to the action of sea water in its surface. 

 Tullberg (1882) further believed that the stem was enveloped with a rind, but Williamson 

 (1907) pointed out that this was true only where the base of the threads enveloped it. 

 The covering was nothing more than the numerous threads which were looped about it, 

 for no rind was present where they were absent. 



The attachment plates, or what Williamson calls the "buttons," are at the distal 

 ends of the threads and serve as the direct medium of attachment for the byssus. They 

 are gray in color and when stained with Bordeaux red or hematoxylin they show a 

 typical alveolar structure with a byssal thread spreading out and terminating in the 

 center. (See fig. 139, opp. p. 158.) 



PHYSIOLOGY. 



The foot serves as an organ of locomotion, and in conjunction with the glands of 

 the byssus cavity it functions in producing the byssus and attaching the threads to favora- 

 ble positions. 



As a locomotor organ it is very effective in performing its functions, although 

 considered degenerate anatomically. The author's attention was directed to the unusual 

 locomotor powers of young mussels when he placed an incrusted mass of material covered 

 with mussels of all sizes from 1 to 50 mm. in length in a glass battery jar under a tap 

 of running sea water. Twenty -four hours later the very young mussels, which measured 



