6 C. S. MINOT ON DISTOMUM 



tical muscular fibres. The basement membrane of the body passes over its free surface, 

 and there is another basement membrane separating the sucker interiorly from the paren- 

 chym (see fig. 11, b.m). The upper and the lower surfaces of the sucker are nearly par- 

 allel with each other, therefore the vertical muscular fibres are almost of uniform length. 

 They run somewhat irregularly, and seem often to bend around at their ends, as may be 

 observed in the sucker of Mesodiscus, 1 Taenia, 2 etc. There are only very few circu- 

 lar fibres, and I have not noticed any radiating ones. The suckers of all Plathelminths 

 that I have examined are -characterized by the great predominance of the vertical fibres, 

 differing in this respect from those of the leeches, which are formed chiefly by circular and 

 radiating fibres. 3 Between the fibres in D. crasslcolle there are numerous rather large 

 oval nuclei,, with a very darkly stained nucleolus, which is highly refringent and excentri- 

 cally placed. To what sort of cells these nuclei belong I cannot make out. They are 

 mostly congregated in the upper part of the sucker. 



The oral sucker (fig. 10, M), as seen in longitudinal section, presents the same histo- 

 logical appearance as the ventral sucker. The digestive canal passes through it a little 

 below its middle, and descending obliquely forwards it reaches the mouth. The vertical 

 fibres are here placed perpendicularly to the axis of the digestive tube. In life, however, 

 the sucker can be everted, so that what forms the beginning of the intestinal canal when at 

 rest, is spread out so as to form the flat outer surface of the sucker. Under these circum- 

 stances the arrangement of the fibres corresponds precisely to that found in the ventral 

 sucker ; we may therefore conclude that the primitive form is that preserved in the ventral 

 disc, while the oral sucker has undergone a secondary alteration producing its present 

 shape. 



Immediately behind the sucker of the mouth there follows a short division of the intes- 

 tine before we reach the pharynx (fig. 10, a). The shape of this part depends upon the 

 position of the pharynx ; when that organ is drawn back the division in question is drawn 

 out to a narrow straight tube ; when the pharynx is pulled forward the tube is shortened, 

 and bulges out, as drawn in fig. 10. This prcepharynx exists probably in all Trematods, 

 its use being to permit the free play of the pharynx backward and forward. 



The pharynx (fig. 10, ph.) closely resembles the oral sucker in its minute anatomy, but 

 is very much smaller, and the oval nuclei are most numerous among the posterior fibres. 

 Up to this point the axis of the digestive canal has been straight. 



Just behind the pharynx (cf. fig. 10) it bends suddenly upwards, and ascends to the 

 dorsal side of the body, and then curves backward and runs without again changing its 

 direction or character, directly to the point where the fork of the intestine branches (cf. 

 fig. 7). This part may be called the oesophagus. It is provided with an internal circular 

 muscular coat, and an external longitudinal one, as may be easily seen either in longitudi- 

 nal (figs. 10 and 12) or transverse sections. Each coat is composed of a single layer of 

 delicate fibres. 



Up to where the intestine branches, I have found it lined by the inflected continuation of 

 the basement membrane of the body, but without any epithelium ; which probably exists, 



1 Minot. Semper's Arbeiten. Bd. in, Taf. xvm, Fig. 3G. Zeitschr. f. Wiss. Zool., 1873. Taf. ix, Fig. 2, D. 

 2 Nitsche, H. Untersuchungen tiber den Bau der Taenien 8 Leuckart. Parasiten. Bd. i, p. 646. 



