MUSCLES. 135 



covered with a thin, moist membrane, and it is no less essential that many 

 animals should be provided with a firm outer covering as a protection against 

 mechanical injury or desiccation. Hence the outer surface becomes more 

 or A'SS distinctly differentiated into two parts, viz., a protecting part, the 

 general integument; and a respiratory part, which is usually preserved 

 from injury by being folded into the interior as in the case of lungs or 

 trachea?, or by being covered with folds of skin as in the gills of fishes, 

 lobsters, etc. 



Motor System. The movements of the earthworm as a whole 

 (locomotion) are produced by the action of muscles forming part 

 of the wall of the body. But besides these muscles there are 



*/ 



many others, of which the most important are those which move 

 the sette (p. 122), the muscles of the dissepiments (p. 159) and of 

 the pharynx (p. 125), the muscles of the nerve-cord and those by 

 which the various peristaltic contractions are performed, the most 

 noteworthy being those of the alimentary canal, of the blood- 

 vessels, and of the nephridia. All of these muscles belong to the 

 motor system, which, therefore, does not form an isolated system, 

 but is intimately connected with many others. 



The muscles of the body-wall are arranged in two concentric layers 

 below the skin. In the outer layer the muscles run around the body, and 

 are therefore called circular muscles. Those of the inner layers have a 

 longitudinal course, i.e., parallel with the long axis of the body, and are 

 arranged in a number of different bands. The most important of these 

 are: 



1. The dorsal land (see Fig. 87), covering the entire back, and extend- 

 ing as far clown on either side as the outer row of seta?. 



2. The ventral bands, covering the ventral side between the two lower 

 (i.e., inner) rows of seta?. These vary greatly in different parts of the 

 body, and in some parts there are additional narrow bands. 



The setae, which must be reckoned as part of the motor system, are pro- 

 duced by glandular cells covering their inner ends, and they grow constantly 

 from this point, somewhat as hairs grow from the root. After being fully 

 formed, and after a certain amount of use, the setae are cast off and replaced 

 by new ones which have meanwhile been forming. In each group we find 

 therefore setae of different sizes. At their inner ends they are covered by 

 a common investment of glandular cells which appears as a slight rounded 

 prominence when viewed from within. These prominences are called the 

 setigerous glands. When a worm is laid open from above, the glands are 

 seen in four parallel rows, two of which lie on either side of the nerve- 

 cord (see Fig. 80). 



