PEDAL GLANDS. 
197 
walls project into the lumen of the body, as a multitude of rauiifyiug 
tubules and follicles. 
The Caudal gdaiids are often found in terrestrial Gastropods at the 
upper surface of the posterior end of 
the body, and form a sinus or cavity, 
such as is present in Hyalinhi 
and very conspicuous in Arion. 
In the foreign genera the gland 
is sometimes distinguished by the 
development of a more or less 
striking pr-otnberance. Its secre- 
tions, which are very abundant and 
much denser than those from the 
skin, accumulate in the more or 
less triangular caudal cavity. 
The Pelecypods sometimes possess on the posterior median line 
an orifice which is homologous with the Ventral pedal sinus of the 
Gastropods, and communicates with a byssal cavity in which are 
gathered the secretions of the foot glands, which have permeated 
the epithelial cellules lining the byssal cavity, and harden on ex2)osure 
in elastic adhesive filaments, whose aggregation forms the byssus and 
serves to attach the animal, as in Dreissensla. The byssal gland 
varies greatly in development in different genera and is often present 
in the young stage when absent in the adult. The calcareous plug of 
Anomia is a modified byssus. 
In addition, there are in some species two other glandular orifices, 
one anterior and the other posterior to the position occupied by the 
vestigial byssal aperture, which it is possible may represent the 
anterior and posterior mucous glands of the Gastropoda. 
Although mucus is emitted from all parts of the body, that secreted 
by the Supra-Pedal gland is of the greatest consistency, and in the 
nude species especially, is exuded so plentifully as to leave an 
iridescent silvery or tinted mucous trail which marks the course the 
animal has travelled. This mucus hardens very quickly on exposure 
to air or water, becoming very tenacious and firm, and is utilized by 
some species as a means of locomotion and for descent from or ascent 
to an elevated position. 
Fig. 389. — The posterior dorsal aspect of 
Hyalinia. cellaria.^ contracted after scalding, 
to show the Caudal INIucus cavity, and in- 
cidentally the rugosities of the body, the 
fringe and the peripodial groove, X 16. 
fr. fringe ; vi.gl. caudal mucous gland ; 
p.f. peripodial furrow. 
