48 AN INTRODUCTION TO ZOOLOGY. 



nephridia, or little kidneys ; or, from their position 

 in successive segments or rings, the segmental 

 organs. These are regarded as a primitive type of 

 excretory organ, from which the kidneys of other 

 types, including vertebrates, have been formed by 

 various modifications. In bivalves there is a paired 

 kidney called the organ of Bojanus, and in the lobster 

 there is a paired kidney called the green gland, which 

 is remarkable for its anterior position, being situated 

 near the front of the head. 



While there are some of the tissues of the animal 

 body that can be satisfactorily examined under the 

 microscope in a fresh state when divided into minute 

 portions, there are others that require to be subjected 

 to the action of various reagents before their structure 

 can be clearly, shown. By means of a razor, fixed in 

 a machine called a microtome, tissues which have been 

 suitably prepared are divided into transparent sections 

 thin enough to be examined under the microscope. 

 The knowledge of these processes and their results 

 constitutes the science of Histology. 



The whole bodies of minute animals are also in the 

 same way divided into sections, and mounted so that 

 they are preserved from change, and can be studied at 

 leisure. Some knowledge of these processes is neces- 

 sary for the student of Zoology ; the beginner, who is 

 ignorant of them, may as a first step to knowledge 

 obtain ready mounted sections of tissues, and of 

 various animals, for examination under the microscope 

 (see Part III. cbnp. IL). 



