SEA MUSSEL MYTILUS EDULIS. 1 59 



the mantle to the shell, and the powerful retractors centered in the foot and byssus, which 

 is the anchoring organ, are so firmly embedded in the calcareous walls of the shell that 

 they can not be separated without tearing the muscles themselves. This enables the 

 sea mussel to thrive in situations where no other shellfish can exist. 



FOOT AND BYSSUS. 



ANATOMY AND HISTOLOGY. 



The foot is a muscular, glandular organ, tonguelike in form, with a deep longitudi- 

 nal groove on the underside that terminates near the tip in a cuplike depression which 

 serves as a sucker when the animal takes hold of a solid object. Posteriorly the groove 

 becomes continuous with the byssus pit (fig. 118, VGr and ByP, opp. p. 138). When the 

 foot is in a contracted state the groove forms an irregular line and its lips have crenated 

 edges. As the foot becomes relaxed and extended, the groove and lips assume a straight 

 form. The base of the groove is enlarged to make a closed canal leading from the 

 byssus pit to tip of the foot when the lips are pressed together. This condition is readily 

 seen in a cross section of the foot (fig. 145, VGr, opp. p. 159). 



The entire surface of the foot is covered with a columnar, dark brown, pigmented, 

 ciliated epithelium, the ciliated parts extending over the inner walls of the groove. 



The portion of the foot which lies immediately below the epithelial covering is made 

 up of numerous muscle bundles which have been described under the muscular system. 

 Between the bundles numerous large blood spaces occur, as may be seen in a cross sec- 

 tion of the organ (fig. 145, BS). 



The central and ventral portions of the foot are filled with a mass of glandular 

 tissue. Tullberg (1882) and Williamson (1907) have made quite thorough studies of 

 the anatomy of the byssus glands and their results and terminology are used in the fol- 

 lowing description. 



The byssus glands may be divided into two sets according to the region they occupy, 

 (1) those of the foot and (2) those of the byssus pit which lies just behind the foot. 



Two kinds of glands are distributed in the foot. The principal one is white in 

 color and is therefore known as the white gland. It is of large size and occupies the 

 middle region of the foot, inclosing the basal canal and extending more or less over the 

 walls of the foot groove (fig. 145, WG). The white gland borders the groove for its 

 whole extent and posteriorly continues backward to surround completely the byssus 

 pit. The second type of the gland in the foot is known as the purple gland. It lies 

 dorsal to the white gland and discharges its secretions into the cuplike depression at 

 the end of the groove. It lies chiefly in the anterior region where it becomes much 

 larger than the white gland. (See fig. 144, PrG, opp. p. 159.) 



The glands of the byssus pit are also two in number. One set, as just described, 

 represents a prolongation of the white gland which surrounds the opening of the byssus 

 pit. Separated masses of glandular tissue of the same nature as the white gland occupy 

 spaces between the muscle bundles and connective tissue of the walls of the byssus 

 cavity (fig. 142, WG, opp. p. 159). The second set of glands is scattered through a series 

 of thin lamellae which are suspended from the dorsal wall of the byssus pit. They run 

 parallel to the long axis of the body and hang down like the leaves of a book (fig. 142, 

 ByG). 



