. ——— 
THE LAMELLIBRANCHIA 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 
ai 
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Fia. 198. 
Fic. 197. Transversal section of a groove of the 
byssogenous cavity of Modiolaria discors. 
Transversal section of the foot of Lyonsia, I, byssogenous glands; II, epithelium of 
through the byssus-orifice. I, byssogen- the byssogenous cavity; III, roots of the 
ous glands ; II, byssus-cavity ; III, byssus- byssus; IV, secretion of the byssogenous 
orifice of the foot ; IV, byssus; V, roots of glands passing between the epithelial cells. 
the byssus. (After Cattie.) 
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 
vit 
Fic. 199. 
Arca lactea, 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, Arca (Figs. 188, 199), Mytilus, Pinna, Avizula 
(Fig. 236), Pecten, various Myacea (Saaicava, ete.), Anatinacea 
(Lyonsia), Cardiacea (Tridacnu), Dreissensia, ete. In the genus 
Anoma the byssus is of peculiar form, being partly calcified and of a 
