SIPHUNCLE AND PALLIAL VESSELS. 755 



an appendix of the visceral sac, that is to say, of the visceral portion of the mantle, 

 and that it owes whatever phj'siological importance it may possess to the blood which 

 is supplied to it from the pallial circulation. While describing the latter we are 

 necessarily led to speak of the siphuncle, which is a vascular vermiform process of the 

 mantle and hence the reason for their association in one chapter. 



By means of injections practised upon fresh specimens when I was living on the 

 shores of Blanche Bay, I obtained a fair knowledge of the distribution of the pallial 

 arteries, such in fact as had not been possible to my predecessors working ^vith pre- 

 served material only. I published an account of the results of my injections in 1896 

 {Quart. Journ. Micr. Sc, Vol. 39), which I propose to recapitulate here, the more im- 

 portant, in my own opinion, relating to the vascularisation of the siphuncle, which does 

 not, as was formerly supposed from the time of Owen's memoir (1832) to that of 

 Dr Haller's memoir', published in 1895, receive a main artery direct from the heart 

 but merely a secondary and two or three tertiary ramifications from the postenor pallial 

 artery. 



Looked at from below through the transparent wall of the mantle the heart is seen 

 lying in the pericardium in the form of a transversely elongated somewhat oblong body 

 placed immediately behind the level of the mantle-insertion and receiving at its four 

 comers the paired branchio-cardiac or efferent branchial vessels (PL LXXXII.). The 

 hinder surface of the heart is slightly convex, while the anterior surface is emarginate, 

 and from the base of the groove there issues a very short vessel called by Owen the 

 " lesser aorta," which he correctly described as dividing almost immediately into two 

 branches, of which he thought the anterior was " exclusively distributed to the mucous 

 organ of the oviduct" (1832, p. 36), by which he meant the nidamental gland, while 

 the posterior passing backwards between the ovary and the gizzard entered, " without 

 diminution of size, the membranous tube that traverses the partitions of the shell." 

 A third branch (intestinal artery) arising after the bifurcation of the lesser aorta was 

 correctly described by Owen as being distributed to the mesentery between the ixscending 

 and descending limbs of the last loojo of the intestine. 



The two primary branches of the lesser aorta are respectively the anterior 

 and posterior pallial arteries, the former supplying the anterior free mantle fold with 

 arterial blood, and the latter irrigating the posterior or visceral portion of the mantle, 

 including the siphuncle. 



The anterior pallial artery bends at first inwards and downwards to the middle 

 line, and then runs forwards below the skin at the surface of the renal sacs. At the 

 point where it turns forwards a small intestinal branch (t.a.) is given off. Arrived at 

 the anterior limit of the region of the renal sacs, which, as already mentioned, forms 

 part of the free mantle-skirt, the anterior pallial artery passes into the substance of 

 the mantle and runs forwards towards the free margin of the latter. At some distance, 

 13 or 14 mm., from the free margin of the mantle, the anterior pallial artery divides 



' Haller, B., " Beitrage zur Kenntniss der MorpholoRie von Nautilus pompiliua" in Semon's Zool. 

 Fornchungsreisen in Australien und devi Malaijischen Arcliipel., Bd. v.. Lief. ii. 1895, see p. 200, and Taf. xi. 

 fig. 2. 



w. VI. 99 



