16 ZOOTOMY. 



46. The abdominal pores (Fig. 5, y), small apertures situated one 

 in each lateral wall of the sinus (the left will probably have been 

 destroyed in opening the latter), and establishing a communication 

 between the sinus and the body-cavity, through which the generative 

 products escape. 



VII. Open the intestine by a longitudinal incision along its whole 

 length : note 



47. The spiral valve (Figs. 4 and 7, int), a longitudinal fold of 

 mucous membrane, projecting into the cavity of the intestine, and 

 having a slight spiral twist. 



48. The aperture of the ossophagus (Fig. 4, y), a longitudinal 

 valve or slit, a short distance from the anterior end of the gut, and on 

 its right side : in front of this aperture, the intestine is produced for- 

 wards into a short thick-walled, blind pouch, situated to the left of the 

 posterior end of the gullet and of the sinus venosus ( 71). This will 

 probably be seen better at a later stage ( 59). 



49. In P. marinus, the aperture of the bile duct (Fig. 4, b.d), on the 

 ventral wall of the gut, just at the point where the latter becomes free 

 from its attachment to the liver. The bile duct is absent in the adult 

 P. fluviatilis, and is not usually present in P. marinus,- though very dis- 

 tinct in the specimen figured. The spiral valve begins just posterior to 

 this point. 



VIII. Turn the animal to the supine position (ventral 

 aspect uppermost), and see the relations of the 

 chief organs from the new point of view. Then 

 remove the ovary or testis, the kidneys, and all 

 but the anterior inch or so of the intestine : make 

 out now 



50. The cardinal veins (Figs. 4, 6, and 7, cd\ two large 

 vessels, lying one on each side of the middle line in the 

 dorsal wall of the body-cavity. 



51. The dorsal aorta (Figs. 4, 6, and 7, d.ao\ a small 

 vessel, lying in the middle line between the two cardinal 

 veins. 



52. Traced backwards, the cardinal veins are found at the posterior 

 end of the body-cavity to unite into a single trunk, or rather to 



