1894 THE MICROSCOPE. 69 



the blood in the gill. The gills are soft bodies of a loose 

 cellular structure supported by the branchial arch. 



In a few species of fish (e. g. Lepidosiren,) the swim- 

 ming bladder seems to act as a lung. It is filled with 

 a spongy cellular structure, and the blood is passed 

 through this and aerated as in the lungs of mammals. 

 But in most cases this is not so. In most fish the ductus 

 pneumaticus is permanently closed and as this is the 

 only passage into the swimming bladder it precludes 

 the possibility of that organ being used as a lung. 



5. The chief use of this bladder is not then as a lung. 

 What effect it has upon the swimming power of the fish 

 is not positively known. It may be that by its con- 

 traction or expansion the fish can determine the depth 

 to which it shall sink. It is on the dorsal wall of the 

 body cavity, above the stomach. 



6. Just back of the swimming bladder and also dorsal 

 in position is the kidney. This is a dark red, spongy 

 organ, and connected with the anus by a straight duct. 



7. The muscles of the back are very strong, while 

 those of the fins are weak. As the fish swims by flexion 

 of the back, and uses the fins mainly for balancing, this 

 development would be natural. In mammals, which use 

 the limbs for locomotion, the contrary is the case. The 

 muscles are made of sets of fibres, and the individual 

 fibres are composed of still smaller parts called fibrils. 

 They are composed as if successive short lengths had 

 been laid down in the process of growth. The whole 

 is contained in a very delicate membrane — the sarco- 

 lemma. The successive growths give a striated appear- 

 ance to the fibril. The high power glass must be used 

 to distinguish these facts. Close to the wall of the fibrils) 

 at nearly equal distances from each other, are nuclei, 

 from which threads of protoplasm extend throughout 

 the fibrils. 



8. The brain is composed of the following parts : the 



