SIPHUNCLE AND PALLIAL VESSELS. 



757 



The posterior pallial artery runs backwards, crossing over the ventral surface of the 

 heart and leaving the pericardium through the median orifice of the pallio-visceral 

 ligament. It then passes backwards without branching on the left side of the gonad 

 between the latter and the gizzard, adhering to the gonad by a ligament named by 

 Haller the genito-intestinal ligament. This ligament binds the gonad and intestine to 

 the posterior wall of the perivisceral coelom with an insertion into the wall of some 

 20 mm. in length running in a dorso-ventral direction. At a site near the middle 

 of the line of insertion of the genito-intestinal ligament into the posterior body-wall 

 occurs the root of the siphuncle. The posterior pallial artery reaches the posterior body- 

 wall through the intermediation of the free ventral border of the ligament at a point 

 distant by 5 mm. from the root of the siphuncle. Upon reaching, in the way described 

 above, what is appro.ximately the middle point of the posterior rounded surface of the 

 body, the posterior pallial artery passes into the integument and immediately divides 

 into two main branches which supply the dorsal and posterior regions of the mantle, 

 including the siphuncle. The two main branches of the posterior pallial artery are 

 the pallio-septal arteries, so called because they supply that portion of the mantle which 

 secretes the septa of the shell. 



Fios. 4 and 5. N. pompilius, i . Two views of the dorsal surface of the pallio-visoeral region, to show the 



principal ramifications of the posterior pallial artery. The siphuncular artery (a.a.) has a dextral origin 



in Fig. 4, sinistral in Fig. 5. 

 Other letters : — m.e. free edge of the dorsal and umbilical folds of the mantle ; c. region of crop ; s.p. siphuncle ; 



I. region of liver; p. p. a. posterior pallial artery; yiz. region of stomach; i/. region of gonad; i. region of 



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



A variable number of small arteries are distributed to the siphuncle, but there is 

 one branch, a secondary branch of the posterior pallial artery, which may be regarded 



99—2 



