338 DIVERSIONS OF A NATURALIST 



by what is called " striated " muscular tissue (as are 

 those of the blood-heart), and they are under the control 

 of branches of the spinal nerves. The movement of the 

 hinder pair in a frog can be seen through the skin. 



In man and all vertebrate animals the intestines, 

 stomach and liver, heart and lungs (or swim-bladder) lie 

 loose, except for a fibrous band of attachment, in a great 

 cavity (often divided into two or more chambers), which 

 they fit fairly closely. The small space between them 

 and the walls of the cavity is occupied by a liquid. 

 This is lymph, and the great cavity is a lymph-space. 

 When this cavity is in its primitive form it is called the 

 body cavity, or " ccelom." In man and mammals it is 

 divided into four chief chambers the peritoneal cavity 

 (in which the stomach, intestine, and liver are loosely 

 attached and have a certain mobility), the right pleural 

 and left pleural cavity (one for each lung), and the 

 pericardial cavity (for the heart). These great chambers 

 are part of the lymph-system, and so is the lymph- 

 holding space around and within the brain and spinal 

 cord, and so are the great spaces beneath the frog's 

 skin. 



If we look at the structure of an earth-worm or of 

 one of the graceful marine worms (Nereis or Arenicola), 

 we gain a good deal of light as to the nature of the 

 lymphatic system of Vertebrates. Suppose you have 

 killed a large earth-worm with chloroform ! Then pin it 

 out on a cork plate, and open it by a cut along the back 

 with a fine pair of scissors. The point of your scissors 

 passes through the muscular body- wall of the worm into 

 a great chamber filled with a clear liquid. This chamber 

 is the " coelom," and is the same structure as the pleural 

 and peritoneal chambers of the Vertebrate. But it holds 



