28 



twenty pairs of muscles can lie made out in connection 

 with the mandibles and the first and second maxillte ; the 

 jaw muscles have been described in detail by Folsom for 

 the CoUembolan OrcheseUa, and the reader is referred to 

 his paper (9). 



Excepting the circular muscle bands of the walls of 

 the heart, the fibres of both the somatic and splanchnic 

 muscles of Anurida belong to the striated variety through- 

 out. The principal muscles of the body consist of a large 

 number of very small fibres, and the latter do not exhibit 

 any tendency to be grouj)ed together into bundles. When 

 view-ed in transverse sections (Plate lY., fig. 45) the central 

 portion of each muscle is seen to be composed of a large 

 number of fibres, which appear as deeply staining angular 

 fields separated from one another by interstitial proto- 

 plasm. The periphery of the muscle is formed of a thick 

 layer of protoplasm, in which are situated one or more 

 relatively large, oval nuclei. The whole muscle is 

 invested externally by a limiting membrane or 

 perimysium. In preparations treated with Heidenheim's 

 iron-alum-liaematoxylin the minute structure of the 

 muscles can be made out with great clearness, and in this 

 resjDect the Collembola are very favourite subjects for the 

 study of the histology of this tissue. The typical light 

 and dark transverse stripes are seen w^ith extreme 

 clearness ; the dark bands or " sarcous elements " are sub- 

 divided longitudinally into extremely fine lines, which are 

 the interspaces between the fibrils or sarcostyles of the 

 muscle (Plate YL, fig. 60). Each light stripe is seen to 

 be bisected by an irregular transverse line [h. m.) which 

 indicates the position of the transverse or Krause's 

 membrane ; the segment of a sarcostyle contained between 

 two transverse membranes constitutes a sarcomere or 

 " muscle segment." By careful focussing by means of a 



