THE LAMELLIBRANCH1A 



217 



maximum of specialisation the byssogenous cavity presents a number 

 of internal folds or plates (Fig. 197) which increase the secreting 

 surface; the trunk of the byssus is thick (Fig. 199, IV), is formed 



FIG. 197. 



Transversal section of the foot of Lyonsia, 

 through the byssus - orifice. I, byssogen- 

 ous glands ; II, byssus-cavity ; III, byssus- 

 oritice of the foot ; IV, byssus ; V, roots of 

 the byssus. 



Fio. 198. 



Transversal section of a groove of the 

 byssogenous cavity of Modiolaria discors. 

 I, byssogenous glands ; II, epithelium of 

 the byssogenous cavity ; III, roots of the 

 byssus ; IV, secretion of the byssogenous 

 glands passing between the epithelial cells. 

 (AfterCattie.) 



more or less deep in the mass of the foot, and becomes engaged in 

 a semi-cylindrical groove hollowed out along the ventral keel of the 

 foot in front of the orifice of the byssogenous cavity (Fig. 236, b.gr). 

 The walls of this groove contain a considerable number of large 

 unicellular mucous glands. The byssus is particularly well 



FIG. 199. 



Arm larttct, Linnaeus, left-side view (the left half of the mantle removed). I, anterior 

 adductor; II, labial palps ; III, foot; IV, byssus ; V, ventral edge of the internal left gill-plate ; 

 VI, right gill ; VII, anus ; VIII, mantle ; IX, posterior adductor ; X, branchial axis ; XI, direct 

 lamina of the external gill-plate ; XII, reflected lamina of the external gill-plate. (After 

 Deshayes.) 



developed in Anomia, Area (Figs. 188, 199), Mytilus, Pinna, Aricula 

 (Fig. 236), Pecten, various Myacea (Saxicava, etc.), Anatinacea 

 (Lyonsia), Cardiacea (Truktcna), Dreissensia, etc. In the genus 

 Anomia the byssus is of peculiar form, being partly calcified and of a 



