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 foes. 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 tyssus, 

 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 luminous 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 air 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 



