\\ITII THE MICROSCOPIC. 159 



253. Ti-aciicse, or air tubes which are characteristic of the class of 

 insects may be demonstrated very readily. If the inside of a common 

 maggot, caterpillar, or fly be removed and covered with thin glass, 

 numerous exceedingly fine dark lines will be seen ramifying almost 

 everywhere, and forming networks. These are the tracheae,and their 

 black appearance is due to their containing air which refracts the 

 light very differently to the other tissues and fluids by which it is sur- 

 rounded. The explanation has been already given in the case of the 

 lacunae and canaliculi of bone, in p. 78. But in this rough mode of 

 examining the tracheae, the student learns nothing concerning the 

 elaborate structure of these air-tubes. If some of the larger ones be 

 dried and then mounted with turpentine or Canada balsam, it will be 

 found that a spiral thread is closely coiled around every one of the 

 tubes, by which beautiful arrangement all are kept pervious, so that 

 air may circulate freely through them, and thus reach the ultimate 

 constituents of all the textures of the body. 



The tracheae all open upon the surface of the . body, by orifices 

 termed spiracles, easily found in the common caterpillar, as they form 

 a row on each side of the body. Every spiracle is guarded by a comb- 

 like arrangement of firm chitinous or horny tissue which prevents 

 foreign particles from passing into the trachese, while like a sieve it 

 permits the free ingress and egress of air. The student should 

 mount a series of specimens of spiracles from different insects. 



254. Braiichite of Moiiusca. The textures of which this form of 

 breathing apparatus is composed may be demonstrated according to 

 the principles already laid down in the sections upon the tissues and 

 organs of the higher animals. The arrangement of the vessels may 

 be displayed by injection, p. 103. The method of demonstrating the 

 circulation of the blood in these organs has been described in page 

 134. In many young mollusks the branchiae are very beautiful. 

 The action of the cilia with which the vessels are clothed is referred 

 to in p. 161. 



255. Microscopic Shells from beautiful objects for investigation ; 

 many may be mounted as dry objects, and examined by low powers. 

 The remains of the animal organisms may be removed by boiling for 

 a few seconds in a weak solution of potash or carbonate of potash. 

 The shells must then of course be thoroughly washed in successive 

 portions of distilled water. 



The shells of foraminifera, many of which are to be found 

 upon our ordinary sea weeds, may be prepared in the same way. 

 This class of organisms has recently been very carefully investigated 

 by Dr. Carpenter, whose beautiful memoirs in the Phil. Trans, 

 are worthy of attentive study. See also " The Microscope and its 



