130 MUSCULAR SYSTEM. 



of those of the lowest animals, before they assume the red 

 colour, the dense fibrous structure, and the highly irritable 

 and contractile property, which they possess in their mature 

 form. 



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SECOND SECTION. 



Organs of Motion in the Cyclo-Neurose, or Radiated 

 Classes. 



Although the aid of the microscope has not enabled us to 

 detect muscular fibres in the transparent and active bodies 

 of the polygastric animalcules, most of them exhibit distinct 

 signs of lively irritability in contracting bending and stretch- 

 ing their whole body, or its anterior prehensile part ; and they 

 are carried rapidly to and fro in their dense element by the 

 rapid vibration of minute cilia generally disposed in regular 

 series on their external surface. The cilia are the organs of 

 respiration, as well as of locomotion in these animalcules, 

 and they are generally longest and largest on the anterior 

 part of the body, especially around the mouth, and in the flat 

 forms, as the cyclidium, the long cilia form a vibratile zone 

 around the inferior margin of the body. Where the ante- 

 rior part of the body is obliquely truncated, as in many of 

 the kolpoda, the vibration of the cilia gives a revolving mo- 

 tion to the whole body in its progression. In most of the 

 trichodae and paramtecia the cilia are disposed in regular 

 close longitudinal series extending over the whole outer 

 surface of the body, and of the greatest length around the 

 oblique mouth. The vorticella have two rows of cilia dis- 

 posed around their anterior circular extremity. The cilia 

 have been observed in almost every class of animals from 

 the polygastrica to the mammalia, and both on the surface 

 and in the interior of the body ; but the means by which 

 they are moved have not been detected. No cilia have been 

 seen in the spermatic animalcules, which are extremely 

 minute and are moved by the lateral motions of their slender 

 tail, like tadpoles, which implies greater irritability and 

 muscular development than in the ordinary polygastrica. 



