114 



COMPARATIVE ANATOMY 



CHAP. 



The complete byssus apparatus (Fig. 102) consists of : (1) a cavity 

 in the foot, into which the byssus gland opens ; (2) a duct connecting 

 this cavity with the exterior ; (3) a groove which runs from the 

 aperture of the duct along the ventral edge of the foot to its anterior 

 end ; and (4) a crescent-shaped or cup-like widening of the groove at 

 its anterior end. 



(1) The byssus cavity is divided into narrow shelves by numerous folds, which 

 project from each side into its lumen. A septum, descending from its roof, further 

 divides it into two lateral parts. The byssus secretion is yielded partly by the cells 

 of the epithelial walls, and partly by glandular cells which lie in the surrounding 

 tissue, their ducts passing between the epithelial cells. The secretion takes the 

 form of the cavity, and is thus held fast as with roots by the numerous lamellse 

 which occupy the shelves. As the amount of the secretion in the cavity increases, 

 these lamellae are pressed into the duct (2), where they unite to form the main stem 

 of the byssus. 



The walls of the groove (3) and its terminal expansion (4) are also glandular. 

 When a bivalve attaches itself it forms a byssus thread 

 in this groove, which fuses with the end of the main 

 stem. The tip of the foot presses against some surface, 

 such as a rock, and attaches the thread by means of a 

 cement secreted by the widened terminal portion (4) of 

 the groove. In this way the main stem of the byssus 

 may be fastened to a rock by means of numerous threads 

 successively secreted in the groove. 



The relation existing between the development of 

 the foot and that of the byssal apparatus may be 

 sketched as follows : 



1. The foot in its primitive form, with a flat sole 

 and no groove, has a simple invagination without 

 byssus (Solenomya). 



2. With the same foot, a small lamella rises from 

 the base of the simple invagination ; the byssus is 

 very slightly developed (Nucula, Leda). 



3. The invagination becomes differentiated into a 

 cavity and a duct, and the byssus and its glands are 

 strongly developed. In consequence of this the foot 

 ceases to be a locomotory organ ; its flat sole 

 disappears, and it becomes finger- or' tongue-shaped, 

 often more or less reduced in size, and serves for 



attaching the byssus. In very many cases the groove is formed from the end of the 

 duct, widening at the tip of the foot as above described. This is especially the case in 

 forms which anchor themselves by the byssus to stones, plants, or the shells of other 

 Molmscs. This attachment may be more or less firm, and may be temporary or 

 permanent (Limidoe, Spondylidce, Pectinidcc, 1 Mytilidce, Arcidce, 1 Carditidcc, 1 Ery- 

 cinidce, Galeommidce, Tridacnidce, Cyprinidce, 1 Fenendce, 1 Glycymcridce, My idee, 1 etc.) 



When the byssus is very highly developed, some of the pedal muscles become 

 attached to the byssus gland and form the retractors of the byssus. 



4. Many Lamellibranchs, in the adult state, have neither byssus nor byssus 

 glands, but the cavity, the duct, and even the retractors (e.g. Trigonia] may be 



FIG. 102. Byssus of a Lamel- 

 libranch with its cavity and 

 duct. 1, Diagrammatic trans- 

 verse section through the foot ; 2, 

 main stem ; 3, terminal threads 

 ie byssus to a foreign 



1 Proparte. 



