414 PHILOSOPHY OF ZOOLOGY. 



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 peculi- 

 ar 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 pigment, the pro- 

 perties of which have not been sufficiently investigated. 

 The animals which furnish tliis 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 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 luminous fluid. Its nature, and 

 the organs in which it is elaborated, have not been inves- 

 tigated. 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 among the animals of 

 this division, chiefly, however, amdng the Conchifera. The 

 most important of these are pearls, so much prized as orna- 

 ments of dress. 



The organs of generation, which will be noticed after- 

 wards in detail, furnish many important characters for clas- 

 sification. The external openings are those which are de- 



