538 



A DESCRIPTIOX OF THE ENTOZOA COLLECTED BY DR WILLEY 



The Alimextary Caxal. 



Ceph.gaAg. 



The mouth lies in the middle of the sucker which is terminal, and opens on 

 the ventral face (Fig. 3, Plate LIV.). The buccal ca\'ity is triradiate, one angle pointing 

 backwards and leading into the narrow oesophagus. Amongst the radiating and circular 

 muscle-fibres in the walls of the suckers were scattered some spherical cells which 

 may have been glandular, but in my opiaion are more probably connective tissue. 

 The fine lumen of the oesophagus is lined by a cuticle surrounded by a circular and 

 then a longitudinal muscle-sheath. 



After a very short course the oesophagus divides into two branches, each of 



which runs towards the edge of the animal and then 

 turns backward, forming one limb of the forked gut 

 (Fig. B in text). The numerous shallow diverticula of 

 these branches and the histological details are fully de- 

 scribed by Walter. Each limb of the gut is somewhat 

 crinkled in its outline. An important point which is 

 shown in the diagram is that the lateral diverticula are 

 all on the outer side of each limb, none pass inwards 

 between the two limbs. They are very regularly ar- 

 ranged and lie in the same plane. The cells lining 

 each limb and the diverticula are low epithelial cells 

 with small nuclei and numerous vacuoles. Owing to 

 the irregularity of the outline of the walls of the 

 alimentary canal its lumen, which in my specimens 

 was always empty, is roughly star-shaped. 



Nervous System. 



Tucked into the angle between the globular pharynx 

 and the slender oesophagus are two half-moon shaped 

 nervous masses (Fig. 3, Plate LIV. and Fig. B in text). 

 They consist of nerve fibrils and ganglion-cells, the 

 latter as a rule nearer the periphery. Slender cords 

 connect the mass dorsally above the oesophagus. Yen- 

 trally and anteriorly a short nerve is given off on 

 each side which runs along the lower surface of the 

 pharynx and soon diwippeai-s. Yentrally and posteriorly 

 a stouter nerve passes backward on each side and nins 

 along the ventral surface of the animal just within the 

 epithelium, giving off transverse branches both exter- 

 nallv and internallv. 



' si. Can 



Fig. B. Diagram of the alimentary 

 canal, nervous sj'stem and water- 

 vascular system of Monostomum 

 trigonocephalum Rud. 

 Al. Can. branches of alimentary 

 canal, Ceph. gang, cephalic ganglion, 

 Ex. Can. lateral excretory canal, water- 

 vascular vessel, N. nerve cord, Sc. 

 sucker, v. vesicle opening at pore of 

 Ex. Can. 



