188 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



either side of the anterior pallial artery, and proceed backward to open into the different bran- 

 chial vessels. At the sides of the mantle there are also a number of lateral pallial veins, which 

 open into a large sinus situated over the shell muscle.*' 



The peculiar perforated structure of the vena cava, together with the large size of the 

 hsemoccel, may have an important bearing upon the ability of the Nautilus to endure being 

 suddenly hauled to the surface without suffering apparent ill results. The specimens which I 

 have had the privilege of studying were captured at a depth of from twelve hundred to eighteen 

 hundred feet. At the latter depth they would be under a pressure of eighty atmospheres. Pro- 

 fessor Worcester has told me that while the Nautili came to the surface uninjured, other 

 animals brought up with them, as fish and Crustacea, were always dead upon reaching the surface. 



It is evident that something in its structure must account for the ability of the Nautilus to 

 withstand such sudden and tremendous changes of pressure, though this function may be. and 

 probably is. only a concomitant of the structure and not its principal function. 



In order that a change of pressure should not prove injurious to an animal it is only necessary 

 that the internal pressure of the tissues should remain equal to the external pressure. It seems 

 to me that this result would be easily accomplished in Nautilus in the following manner. The 

 pressure of the surrounding water upon the body would be transferred immediately to the blood 

 contained in the hsemoccel. The cavity of the hsemoccel is in direct communication with that of 

 the vena cava, and consequently with all the vascular spaces of the body, through the holes in 

 the dorsal wall of the vena cava. By this means the pressure of the blood in the hamiocoel is 

 directly transmitted to the blood of the entire body, and thus the pressure within and without 

 the body is equalized. No change in the volume of the body would occur because the volume of 

 the hsemoccel and ccelom is minimal, and because of the incompressibility of the fluid. 



The hsemoccel is completely closed from the exterior, so no water enters it, or anywhere 

 comes in direct communication with the blood. It seems entirely improbable that water ever 

 enters the ccelom through the pericardial pores, as has been suggested. 



NERVOUS SYSTEM. (FIG. 41.) 



The central nervous system of Nautilus consists of three ganglionic bands which unite so as 

 to form a ring around the (esophagus, two passing ventrallv to the oesophagus and one dorsally. 



The dorsal band represents the cerebral ganglia plus their commissure (12). The ends of the 

 band are sometimes slightly larger than the central portion, but there is never any such separa- 

 tion of the parts as to allow us to say. these are the cerebral ganglia, or this is the cerebral 

 commissure. 



The posterior of the ventral bands represents the pleuro- visceral ganglia (13); this also is not 

 separated into a pair of ganglia and a commissure, although the ends of the band are sometimes 

 larger than the central portion. The anterior ventral band is composed of two distinct ganglia 

 united by a slender commissure, the pedal ganglia (28) and the pedal commissure (29). The 

 pedal ganglia are flat and crescentic in outline. 



The cerebral, pedal, and pleuro-visceral ganglia form a junction at the sides of the oesophagus. 

 The pleuro-visceral ganglia seem almost to join the pedal ganglia rather than the cerebral, but 

 closer examination proves that they unite with the cerebral ganglia to as great an extent at least 

 as with the pedal ganglia. 



I shall speak of the cerebral and pleuro-visceral ganglia as if each band were in reality a 

 single ganglion. 



From each outer side of the cerebral ganglion an enormous optic nerve passes outward into 

 the stalk of the eye (24 and 25). The base of the optic nerve is swollen and may form an optic 

 ganglion, a point which the study of sections alone will settle. The optic nerve is almost 

 immediately divided into numerous parallel small nerves which in passing outward twist slightly 

 about the axis of the nerve. They are much more closely pressed in the stalk of the eye than 

 nearer the cerebral ganglion. At the back of the retina the nerves separate and form a mesh 

 about this bowl-shaped organ. 



Near each end two nerves pass from the anterior side of tin 1 cerebral ganglion forward to 



