oo2 LEWIS HENRY OOUGH. 



of the annulil'oi'in muscles would indicate that the young 

 muscles develop on the outside of the transverse muscle. 



The myoblasts are bipolar, spindle-shaped and very 

 elongate (figs. 26, 27) ; their nuclei resemble in most respects 

 the nuclei of the myoblasts of the other two systems, but 

 they are usually somewhat larger. Those measured averaged 

 4'25/x, to 5 jit long by 2"5^ to 4"25^ broad. ' The plasma is 

 granular; a membrane appears to be present. The ends of 

 tlie cells are inserted iu the tubular ends of the muscles. In 

 some cases the two tubiform muscles appear to be connected 

 by a strip of muscle-tissue passing along one side of the 

 myoblast. Similar muscles were observed by Salensky (1874) 

 in Amphilina, although not re-found by either Hein (1904) 

 nor by Colin (1904) in the same object. 



The longitudinal muscles appear to me to be produced by 

 these cells, which in that case are true myoblasts, the con- 

 tractile substance being formed or deposited on the surface 

 of the cell, the body of the cell penetrating the axis of the 

 muscle for a long time ; then, as the muscle develops, its 

 cavity becomes filled with muscle substance. In the end the 

 cell disappears from the; axis of the muscle, and becomes 

 displaced so as to lie laterally to the muscle. Nuclei lateral 

 to the muscles are frequently observed. 



The muscle-fibres can be traced through a number of 

 segments. Their course is not quite straight, but more or 

 less sinuous. Text-fig. o is a photograph of a section cut 

 parallel to the longitudinal axis of the worm, and represents 

 a distance of about ten segments. 



In transverse sections the muscles appear to be arranged 

 iu bundles, whose elements are more crowded towards the 

 transverse muscle, more scattered towards the subcuticula. 

 However, I distrust the appearance of the bundles, since it is 

 impossible to re-discover them in horizontal sections (see Text- 

 fig. 3). It will be seen that where a few muscles do group 

 together, they receive fibres from the adjoining groups and 

 give off fibres. 



The schematic Text-figs. 4, .">, G will help to make my 



