STRUCTURE OF CLINOSTOMUM 203 



were drawn with the same objective but 11 is made with a higher 

 eyepiece. Such views are found generally in many different 

 series so that we are justified in regarding them as a normal fea- 

 ture of the cytological structure of Clinostomum muscle. In 

 fig. 1 1 it is clearly seen that the muscle is made up of several paral- 

 lel-sided filaments of considerable length, composed of a substance 

 which is not strongly influenced by haematoxylin and a second 

 deeply staining substance. The swollen globular appearance of 

 the latter leads us to believe that it is a peculiar ' contractile sub- 

 stance.' The fibers do not all present this appearance. One is 

 represented in fig. 10 in which also the myoblast and nucleus are 

 shown. The myoblast is large, measuring 0.014 mm. across, and 

 the nucleus has a diameter of 0.005 mm. Fibers can be traced 

 from such myoblasts. These appear differently from those in 

 figs. 9 and 11, showing a dark contour on the wall and a clearer 

 center. In cross sections the appearance is that of two substances, 

 a clearer central and a darker surface material. These seem to 

 be the 'hollow muscles' of writers. Fig. 10 shows the fibers, 

 probably in an uncontracted state, while 9 and 11 are contracted 

 fibers. A more adequate study of the cytology of this muscle is 

 bej^ond the scope of this article. 



THE ALIMENTARY APPARATUS 



Oral sucker. The mouth opening lies in the center of the oral 

 field and leads into the cavity of the oral sucker. This sucker is 

 nearly spherical and is very much smaller than the ventral sucker, 

 measuring 0.28 mm. long and 0.25 mm. across. It has the usual 

 cuticular lining and heavy muscular wall composed of fibers run- 

 ning in the various directions. 



Oesophagus. The pharynx, which is generally present in tre- 

 matodes and usually follows close after the oral sucker, is entirely 

 wanting. There is a short tube immediately behind the oral 

 sucker which, after running ventrally a short distance, makes a 

 dorsal bend to meet a transverse portion of the intestine. This 

 is the oesophagus. The structure of the two bends is somewhat 

 different. The more anterior portion is very thin walled and is 

 lined with a thin cuticle continuous with that of the oral sucker. 



