Medicinal Leech. 129 



the cephalic, and some upon other segments of the bod}', 

 and which, as resembHng- the ' becher-formige Organe^ offish, 

 may be supposed to be sensory in function, and concerned 

 possibly with the perception of chemical rather than of other 

 stimuli, see Leydig, Archiv. fur Anatomic und Physiologic, 

 1861, p. 599; Tafeln zur Vergleich. Anatomic, iii. i ; F. E. 

 Schultze, Zeitschrift Wiss. Zooh xii., 1863, p. 222. 



40. Medicinal Leech {Hirudo Medicinalis), 



Prepared and dissected so as to show its laterally sacculated stomach and its in- 

 testine, in their natural relations to tlie ventral chain of ganglia inferiorly, and 

 the pharynx anteriorly. 



A STIFFENING injection having been thrown into the digestive 

 tube, the specimen was hardened in spirit. After this, the in- 

 tegument having been divided down the middle dorsal line and 

 reflected outwards, the portions of the pseud-haemal system which 

 were interposed between the digestive tract and the dorsal surface 

 were removed, and the entire cavity of the 'stomach^ and of its 

 diverticula exposed by the removal of its upper wall. Anteriorly 

 to the stomach is seen the pharynx with a villous exterior, much 

 resembling that in the earthworm^ as seen by the naked eye, as 

 also when its constituent elements, unicellular gland cells and 

 involuntary muscular fibre, are examined under the microscope. 

 Partly concealed by this villous exterior of the commencement of 

 the digestive tract, may be seen the prae-oral ganglionic mass. 

 The walls of the portion of the digestive tube which comes next 

 after the pharynx, are much thinner than those of the pharynx 

 itself, and consist mainly of a structureless basement membrane and 

 an internal layer of pavement epithelium ; its muscular coat being 

 almost wholly aborted, as in the Ophidia (see p. 30), and its func- 

 tions discharged by the muscular layer of the body-walls. This 

 portion of the digestive tube has a much larger calibre than the 

 pharynx, and has lateral diverticula appended to it on either side, 

 which occupy five-sixths of the entire cavity of the body. These 



^ See Plate viii. b. 

 K 



