MUSCLES. 63 



the form of long bands or sheets of parallel fibres, such as those 

 that form the body-wall, that move the setse, and dilate the 

 pharynx. Other muscular structures, however, do not form dis- 

 tinct " muscles,' but consist of muscular fibres more or less, 

 irregularly arranged and often intermingled with other kinds of 

 tissue. Of this character are the muscular walls of the contrac- 

 tile vessels, and of the muscular portions of the nephridia and 

 dissepiments. It is clear from the above that the muscular sys- 

 tem is not isolated, but is intimately involved in many organs. 



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 bands (Fig. 39), one on either side above, in contact at 

 the median dorsal line, and extending down on either side as far as the 

 outer row of setae. 



2. The ventral bands, on either side the middle ventral line and occupy- 

 ing the space between the two inner (lower) rows of seta?. 



3. The lateral bands, occupying the space on either side between the 

 two rows of seta?. 



All these vary greatly in different regions of the body, and in some parts 

 become more or less broken up into subsidiary bands. There is also a 

 narrow band traversing the space between the two setae of each group. 



The seta, which may be reckoned as part of the motor system, are pro- 

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

 stantly 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 prom- 

 inences 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. 29). 



Each group of setae is provided with special retractor or protractor 

 muscles, and a narrow muscular band passes from the upper to the lower 

 group on each side internal to the body-wall. 



Cilia. A second set of motor organs are cilia (their mode of action has 

 been referred to on p. 31), which are of the utmost importance in the 

 life of the earthworm. They cover the inner surface of the stomach-intes- 

 tine (where they doubtless assist in the movements of the food) play the 

 important part in excretion already described, collect and help to discharge 



