278 C. O. WHITMAN. 



segmpntp, corresponding with the number of postoral ganglia, 

 is thirty-three. Eight are converted into the suctorial disc {d). 



Some of the posterior ganglia are ahvavs rudimentary, and it 

 is rather difficult to obtain preparations which show all the 

 ganglia of the disc region ; hence the general opinion that only 

 seven segments enter into this part. The divisions of the nerve- 

 chain are at first quite alike from end to end. 



The definitive differentiation into four regions is already 

 beginning to show itself, two and a half days after the exclusion 

 (fig. 91). The first four divisions (1 — 4«) are a little broader 

 than the following, and are destined to coalesce more or less 

 to form the sub-cesophageal ganglia.^ The next region includes 

 seventeen divisions (5 — 31), which later stand at considerable 

 intervals from one another, connected by the double longitudinal 

 commissures. 



The third region embraces the last four divisions (22 — 25) 

 of the body proper; they concentrate to a continuous tract of 

 ganglia. The fourth and last region includes the eight divisions 

 (26 — 33) found in the terminal sucker. The concentration in 

 this region will later obliterate the limits of the original ganglia. 

 The dotted transverse lines in this figure mark the position of 

 the septa. The dots represent yolk-grauules contained in the 

 embryonic cells. The preparation is seen from the hsemal side. 

 In the mesial line of each ganglionic mass are two pairs of small 

 cells, the signification of which is unknown. 



VI. Alimentauy canal. 



This consists of four parts (fig. 56) : (1) The protrusible 

 pharynx (p), which ordinarily lies ensheathed in the pharyngeal 

 atrium; (2) a short oesophagus; (8) a sacculated stomach (st.) 

 or crop (Gratiolet), which stretches through the greater part of 

 the body ; (4) a narrow intestine (int.) (" gostroileal^^ intestine, 

 Gratiolet). 



The formation of the pharynx and the pharyngeal cavity has 

 already been considered. The histogenetic origin and structure 

 of the remaining parts will be better understood after a description 

 of surface changes. 



The adult form of the digestive tract is much like that given 

 in fig. 5(). The stomach is divided into three well-marked 

 regions. The small anterior and posterior regions are almost the 

 exact counterparts of each other, each having four jjairs of 

 diminutive lateral sacculations. Those of the anterior part point 

 obliquely forward, and those of the posterior part obliquely 

 backward (exccjjting the foremost pair, which point forward). 

 The main central region has seven pairs of large lateral diverti- 



' IIofTmanii {\],) states that onlj three so coalesce. 



