1895.] AH^ATOMT OF NAUTILUS POMPILIUS. 665 



exterior by apertures in the body-wall. To a body-cavity of this 

 type it is advisable to restrict the term Ccelom. 



The second type of body-cavity is to be found in the Mollusca 

 and Arthropoda generally. It is part of the vascular system, 

 through it is pumped a continuous stream of blood by the heart, 

 and it does not communicate with the exterior. It may be looked 

 on as being formed by the expansion of the terminal parts of the 

 blood-vessels into large sinuses w-hose walls have, to a greater or 

 less extent, disappeared, giving rise to a sponge-w ork more or less 

 sparse according to the extent to which this process has gone on. 

 This type of body- cavity was named by Sedgwick, PseHrfoco?/ ; by 

 Lankester, Hcmocoel. The word ccelom has been used with such 

 looseness that Lankester's term is perhaps to be preferred ; all the 

 more so as it specifies in itself one of the main characteristics of 

 this form of body-cavity. 



Occurring well developed in Annelids, at least allied in all pro- 

 bability 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 tlie 

 ever increasing haemocoel. 



In most Cephalopods the ccelom still takes a large part in the 

 formation of the perivisceral cavity, and in jyautilus, correspondiug 

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

 in any of the other Cephalopods. 



The haemocoel 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 

 haemocoel; ventrally it is bounded by the body-wall and the mus- 

 cular 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 coelom. The 

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

 appearance, and little connective-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 haemocoeUc natm-e 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 communication 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 aboTe I see that Pelseneer iji bis recent ' Etude des Mol- 

 lusques,' p. 191, says that "la cavite visceral est uh raste sinus commujiiqiiant 

 »vec la Teine cave par des orifices perces dans la paroi de celle-ci." 



