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Show 1895.] ANATOMY OP 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 whose walls have, to a greater or less extent, disappeared, giving rise to a sponge-work more or less sparse according to the extent to which this process has gone on. This type of body-cavity was named by Sedgwick, Pseucloccel; by Lankester, Hcemoccel. 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 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, corresponding with its more archaic character, this is so to a greater extent than in any of the other Cephalopods. The baeinocoel 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 haemoccel; 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 tbis septum has a rough and spongy 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 haemoccelic 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 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 observers1. 1 Since writing the above I see that Pelseneer in his recent ' fitude des Mol-lusques,' p. 191, eays. that "la cavite visceral est un vaste sinus communiquant a-vec la veine cave par des prifices perces dans la paroi de celle-ci," |