SIPHUNCLE AND PALLIAL VESSELS. 



761 



free lip of the shell merges into the umbilicus. When a large enough hole has been 

 made in the shell to admit the scissors, the shell being still held upside down, the 

 postero- ventral visceral portion of the body behind the annulus usually detaches itself 

 from the shell, or can be readily caused to do so, and sinking inwards, exposes the 

 root of the siphuncle, which can then be severed. Upon righting the shell the body 

 resumes its normal contact with the fundus of the shell, thus preventing an immediate 

 and extensive hemorrhage. For example a Nautilus treated in this way, and replaced 

 in the sea at a suitable spot, swam about vigorously for some time in the middle stratum 



p.s.a. J 



m.pa. 



post.p.a. 



Fig. 6. JV. pompilius, 3 . Dorso-posterior aspect of visceral region, to illustrate the circulus pallialis and the 

 septal contour ; n.m. nuchal membrane ; n.a. nuchal artery ; m.e. free mantle-edge ; p.n.a. pallio-nuchal 

 artery ; m.p.a. marginal pallial artery ; col. columellar or shell muscle ; p.c.a. posterior columellar artery ; 

 p.p. a. posterior proventricular artery; s.c. septal contour; s. siphuncle; p.s.a. pallio-septal arteries; 

 s.a. siphuncular artery; I. liver; i. intestine; t. testis; post.p.a. posterior pallial artery; g. gizzard. 



N.B. — The dorsal aorta and its branches are indicated by dotted lines. They show dimly through the skin 

 when injected. [From Quart. J. Micr. Sc. Vol. 39, 1896.] 



of water, but chiefly at a little distance from the bottom. Others remained floating and 

 swimming about on the surface during the whole time of observation. They did not 



