ON PHYMOSOMA VAEIANS. 7 



spindle muscle supporting the alimentary canal is shown 

 running up the axis of the intestinal coil. The oesophagus is 

 anteriorly surrounded by the retractor muscles, but the poste- 

 rior half is free and ends in the coiled intestine. The number 

 of coils varies, usually there are about fifteen. The intestine 

 forms a thicker tube than the oesophagus, it ends in the 

 rectum which passes straight to the anus in the dorsal middle 

 line. 



The only part of the vascular system visible is the crumpled 

 dorsal vessel. 



The brain is indicated through the walls of the introvert, 

 and close behind it, at the sides, two black spots, the sense- 

 pits, are visible ; the ventral nerve-cord is seen running down 

 the body. 



The nephridia or brown tubes are conspicuous objects, vary- 

 ing very much in size and shape in different individuals. 

 Their external opening is at the anterior end and a little in 

 front of the level of the anus. The opening is followed by a 

 short neck which opens into the swollen portion or bladder 

 which passes into the true secreting portion. The anterior 

 half of the nephridia is attached to the body wall by muscle- 

 fibres, the posterior is free (fig. 18). 



The generative ridge runs across the body at the base of the 

 ventral retractors (fig. 22). It is sometimes V-shaped, the 

 ridges slanting backward in the middle ventral line. 



The Muscular System. 



The muscular system is composed throughout of fusiform 

 fibres with simple pointed ends. Each fibre consists of an 

 outer contractile and an inner granular portion, the outer por- 

 tion being longitudinally striated. The elongated oval nucleus 

 lies entirely within the inner layer, the nucleus and the con- 

 tractile layer being easily stained, while the inner substance 

 does not absorb staining fluids (figs. 13 and 21). 



The fibres of the retractor muscles are much larger than 

 those of the body wall, their diameter being at least twice as 

 great. 



