MOLLUSCA. 19 
divisions to be employed, it is unnecessary, in this place, to 
enter into their details. 
PECULIAR SECRETIONS.—The molluscous animals are con- 
sidered as destitute of organs for the production of urine, 
but they possess various organs for the secretion of peculiar 
fluids or. solids, some of which are useful in the arts. 
The coloured fluid, which is secreted by the Cephalopo- 
da and some of the aquatic gasteropoda, appears to consist 
chiefly of a peculiar mucus, united to a peculiar pigment. 
The animals which furnish this secretion, eject it when in 
danger or irritated, and thus envelope themselves in a dark 
cloud, and elude the pursuit of their fees. A milky secre- 
tion is‘ poured forth over the surface of the skin of some 
slugs when irritated. Other coloured secretions may like- 
wise be detected in the mollusca, to which we shall after- 
wards advert. The threadlike secretions, termed a byssus, 
with which some molluscous animals, especially among the 
Conchifera, fix themselves to other bodies, appear to be of 
an albuminous nature. A few species in this division have 
the power of secreting a /uminous fluid. Its nature, and the 
organs in which it is elaborated, have not been investigated. 
It is probable that some animals, as those which have the 
faculty of raising or lowering themselves in the water, have 
likewise the power of secreting ar into those organs which 
contribute to their buoyancy. 
Morbid secretions likewise occur amongst the animals of 
this division, chiefly, however, amongst the Conchifera. 
The most important of these are pearls, so much prized as 
ornaments of dress. 
The organs of generation, some of which will be noticed 
afterwards in detail, furnish many important characters for 
