ON THE ANATOMY OF NAUTILUS POMPILIUS. 183 



probability to the ancestral forms of Molluscs and Arthropods, the 

 ccelom is to be looked on as the more primitive of the two types 

 of body-cavity above-mentioned; and it looks as though within 

 each of the two latter groups it had gradually dwindled and 

 become supplanted and replaced as the functional perivisceral 

 cavity by the ever increasing hsemocoel. 



In most Cephalopods the ccelom still takes a large part in 

 the formation of the perivisceral cavity, and in Nautilus, corre- 

 sponding with its more archaic character, this is so to a greater 

 extent than in any of the other Cephalopods. 



The hsemocoel of Nautilus is specially developed in the 

 headward section of the body. A sagittal incision through the 

 body-wall just behind the hood exposes to view a large chamber 

 in which lies the pharynx as well as the vena cava, several 

 large nerve-trunks, and a single loop of the intestine. This 

 cavity is the main division of the hssmocoel ; ventrally it is 

 bounded by the body-wall and the muscular substance of the 

 hood, etc., into which it extends in numerous sinuses, while 

 dorsally and towards the apex of the visceral hump it is 

 bounded by a thin and delicate but complete membranous 

 septum which forms the boundary between it and the ccelom. 

 The inner ("ventral") face of this septum has a rough and 

 spongy appearance, and little coimective-tissue strands pass 

 from it to the surface of the pharynx. These delicate threads 

 of connective tissue traversing the cavity and slinging up 

 its contained organs at once suggest the htemoccelic nature 

 of this part of the body-cavity : and the conjecture is 

 confirmed on raising up the pharynx, for one then sees that the 

 upper wall of the vena cava is perforated by numerous foramina, 

 some of considerable size, which put its cavity into free com- 

 munication with that part of the body-cavity now under 

 discussion. These foramina were described and figured long 

 ago by Owen, in his Monograph, but they appear to have been 

 unnoticed by subsequent observers ^ 



' Since writing the above I see that Pelseneer in his recent Etude des 

 Mollmques, p. 191, says that " la cavity visc&ale est nn vaste sinus communiquant 

 avec la veine cave par des orifices perc^s dans la paroi de celle-ci." 



