188 MEMOIRS OF IHE NATIONAL ACADEMY OF SCIENCES. 



either side of llie anterior pallial artei-y, and proceed backward to open into the ditierent ))ran- 

 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 witii the large size of the 

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

 suddenlv hauled to the surface without suflering 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 WoKCESTER has told me that whil(> the Nautili came to the surface uninjured, other 

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



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

 withstand such sudden iind 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 pre.ssure should not prove injurious to an animal it is only necessary 

 that the internal pressure of the tissues should remain (Hjual to th(> external pressure. It seems 

 to me that this result u'ould be easily accomplished in Nautilus in the following manner. The 

 pressure of the surrounding water upon the body would be ti'ansferred immediately to the blood 

 contained in the hsemocoel. The cavity of the ha-mocrel is in direct conununication with tiiat of 

 the vena cava, and consequently with ail th(> vascular spaces of the body, through the holes in 

 the dorsal wall of the vena cava. By this means the pressure of the ])lood in the hsemoccel is 

 directly transuiitted to the l)lood of the entire body, and thus the pressui'e within suid without 

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

 the hismoccel and ccelom is minimal, and because of the incompressibility of tlie fluid. 



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

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

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



NERVOUS SYSTEM. (FIG. 41.) 



The central nervous system of Nautilus consists of three ganglionic liands wliich unite so as 

 to form a ring around the <esophagus, two passing ventrally to the (esophagus and one dor.sally. 



The dorsal band icpresents the cerel)ral ganglia plus their commissure (12). The ends of the 

 band are sometimes slightly larger tiiaii the central poition. l)ut 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-vi.sceral ganglia (13); this also is not 

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

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

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

 pedal ganglia are flat and crescentic in outline*. 



The cereltral. pedal, and pleuro-visceral ganglia form a junction at the sides of the (esophagus. 

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

 closer examination proves that they unite with the c(>rebral ganglia to as great an extent at least 

 as with the pedal ganglia. 



1 shall speak of the cerebral and ])leuro-visceral ganglia as if eacii l)and 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 s(>ttle. The optic nerve is almost 

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

 about the axis of the nerve. They are much more closely pre.ssed 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 eacli end two niM'ves pass from tiie aiilerior side of the eereliral ganglion forward to 



