MUSCLES 177 



413. Locomotion in Vertebrates is similar to that of Arthro- 

 pods in principle, with this important difference: The levers 

 occupy the axis of the appendage while the muscles are attached 

 to the surface and lie outside. A mechanical advantage is here 

 obtained by the greater flexibility of the joints. The hinge 

 joint, which is the only one possible in Arthropods, permits of 

 motion in one plane only, while the ball and socket joint, which 

 is found at many points in the vertebrate skeleton, gives uni- 

 versal motion. 



414. The muscular tissue of the body- wall and the organs of 

 locomotion, is composed of fibres of a complex structure. 

 Almost the entire substance of the cell is transformed into 

 muscle fibrils, of which there are a large number. There is 

 also a fibre sheath which binds the fibrils together, and among 

 the fibrils are a number of nuclei. This type of fibre is, there- 

 fore, not a uninuclear cell. The most striking characteristic of 

 these fibres is the banded appearance which they present under 

 the microscope. The light is affected differently at different 

 points in the fibre so that some appear light and others dark. 

 These points alternate regularly and give the fibre the appear- 

 ance of being crossed by alternating dark and light bands. Such 

 muscular tissue is called cross striped or striate and distinguishes 

 the skeletal muscles of Vertebrates and Arthropods from the 

 muscles of Worms and most other invertebrates. The heart of 

 Vertebrates is also composed of striate muscle, but the muscles 

 of the digestive tract and many other parts of the body are more 

 like those of the Annelids. They are called smooth muscle 

 fibres. Generally the cross-striped muscles are more quick and 

 vigorous in action than the smooth muscle fibres. See Fig 101. 



SKELETON AND CONNECTIVE TISSUE 



415. Between the ectoderm and entoderm of hydra there is 

 a thin layer called the supporting lamella. It is secreted by the 

 cells of the ectoderm and entoderm, and is, therefore, not a 



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