116 SECRETION. 



are capable of extending their tails to a considerable de- 

 gree, so that they can keep them above the surface 

 without always changing their positions with every 

 change in the level of the water. Reaurner desirous of 

 seeing how far they could thus extend themselves, be- 

 gan to add water slowly till he had raised it six inches ; 

 the larvae in the mean time continued to lengthen them- 

 selves until they reached this height, when they were 

 forced to change their position and attach themselves 

 high up on the sides of the vessel. 



Spiders breathe by means of eight or ten stigmata 

 which lead into a sac, on the sides of which are situated 

 little laminae or fringes, in which the blood circulates 

 that is to be renewed by the air. 



Leeches and earth-worms have no other apparatus 

 for breathing but the skin, in the vessels of which the 

 blood is aerated. 



Emily. What wonderful diversity in the construction 

 and arrangement of organs, whose ultimate purpose is 

 the same ! And how perfectly does every change har- 

 monize with other co-existing modifications ! 



Dr. B. Having finished the subject of respiration, 

 we will turn our attention to the function of secretion. 



Emily. Secretion means separation ; I had no 

 idea that there was any such function as this in the ani- 

 mal economy. 



Dr. B. All the fluids in the animal body are de- 

 rived from the blood, and the process by which they 

 are separated from the blood is called secretion. But 

 you are not to suppose that all these fluids existed 

 already formed in the blood, and that the only object of 

 secretion is to separate them therefrom ; for some of 

 them in deed most of them are formed by the se- 

 cretory organs from materials which exist uncombined 

 with the blood. 



Emily. This process approaches the operations of 

 chemistry nearer than any other which we have yet at- 

 tended to. 



