18 



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. Each 

 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 Cardium possesses a true byssus was demonstrated 

 by Gosse,* who shewed that in the young C. acnleatum 

 this was actually functional. Carrieret and BarroisJ have 



* Ann. and Mag. Nat. Hist. Ser. II., vol. XVIII., pp. 257—8. 



t Arb. Zool.-Zoot. Institut, Wurtzbnrg. Bd. V., pp. 56—92, 1882. 



X Comptes Rendus Acad. Sci, T. C, pp. 188—190, 1885. 



