SEA-FISHERIES LABORATORY. 195 
the surface, indicate their probable nature—that of a 
simple mucus-secreting apparatus. 
The pedal groove itself is not a glandular structure, nor 
do the glands above described open on to its surface. 
Passing backwards from the tip of the foot the cilia dis- 
appear, and the groove is lined with an epidermis consist- 
ing of simple columnar cells. The groove becomes deeper, 
and may be thrown into longitudinal folds. Finally, it 
passes into a duct which runs upwards and backwards 
into the proximal limb of the viscero-pedal mass, and 
terminates in a swollen bulbous head, which lies on the 
right side of the caecal prolongation of the straight intes- 
tine (By.g., fig. 3). A single delicate hyaline fibre may 
sometimes be seen projecting from the opening of this duct, 
and indicates, what the histological character of the gland 
in fact demonstrates, that the apparatus is a byssus secret- 
ing structure. Sections of the duct show only an 
epithelium consisting of short columnar, ciliated cells. 
Further up, the duct expands into a wide cavity (fig. 19) 
from which lateral diverticula are given off; these may 
branch again. Their lumina are always restricted. Hach 
of these secreting alveoli (fig. 20) is lined with a epithelium 
of rather club-shaped cells which do not seem to bear cilia, 
but from between which a number of fine threads pass out 
into the lumen where they become agglutinated together, 
forming a filament. The filaments from the various 
alveoli unite together in the duct to form the single byssus 
thread. 
That Cardiwm possesses a true byssus was demonstrated 
by Gosse,* who shewed that in the young C. acwleatum 
this was actually functional. Carriere} and Barrois{ have 
* Ann. and Mag. Nat. Hist. Ser. II., vol. XVIII., pp. 257—8. 
t Arb. Zool.-Zoot. Institut, Wurtzburg. Bd. V., pp. 56—92, 1882 
¢ Comptes Rendus Acad. Sci. T. C., pp. 188—190, 1885, 
