GLANDULAR SYSTEM — CAUDAL AND IIYPOBRANCIIIAL GLANDS. 319 
Upward tlireads can only be formed by the aqnatic Pulmonates, 
as beino- attached to some submerged object and always directed 
towards the surface of the water, they can only be formed by 
individuals using their air-intlated lung-sac as a hydrostatic organ 
enabling them to rise to the surface of the water, as they are then of 
less specific gravity than the water they inhabit. 
Downward threads may be attached to the surface film of the water 
or to any object elevated above the surface of the mud, in the former 
case the exposed expanded hice of the mucous filament being slightly 
depressed or hollow acts like a boat and siqqmrts the thread 
depending from it. The branchiate Streptoneura, especially when 
yonng, can form the downward threads, Bythinla tentacuhita being- 
observed to be particularly addicted to suspending itself from the 
surface of the water by this means. These filaments can also be 
formed by Pulmonates, which have expelled some of the air from the 
respiratory chamber and thus become heavier than the water they 
inhabit and tend to fall to the bottom. 
The formation of mucous cords is not, however, confined to the 
Gastropods, as they are also made use of by certain of the Pelecypoda, 
especially when young, Sphccrium lamstre being especially noted for 
indulging in this habit of suspending itself, sometimes for hours, from 
the vegetation or ihe surface of the water, by one or more mucous 
threads ; the rate of production of these filaments being recorded as 
three hours for the formation of a filament half-an-inch in length. 
The Caudal Gland is usually constituted by an actively secretory 
area, and a more or less triangular receptacle or cavity upon the upper 
surface of the extremity of the tail, and is present in a variously 
developed form in many diverse genera, but is especially conspicuous 
in the members of the genus Arion. 
In Avion the glandular portion is formed by an aggregation of 
minute brownish, semi-transparent and rounded secretory cellules, 
which combine to form long, irregularly sinuate lobes, separable by 
maceration, the gland secretes considerable (piantities of rather clear 
and tenacious mucus with great rapidity, especially during the 
pairing season, when large and conspicuous globules of glutinous 
mucus frequently accumuhite within the caudal cavity or receptacle. 
The IIypobranchial Gland is a localized and variously shaped 
mucous gland, intersected by nerves and blood vessels, and serving 
for the lulndcation of the respiratory cavity and the oi’gans thei’ein. 
