. Contribution to the natural history of the pearly nautilus. Nautilidae. PERISTOMIAL HAEMOCOEL ; SYSTEMIC AORTA; CEPHALIC ARTERIES. 781 side, when a forest of slender conjunctive trabeculae are found passing from the inner (mesial) surface of the liver to the wall of the crop. The peristomial haemocoel is virtually separated from the peri-oesophageal space by a tendinous diaphragm on the dorsal side, and by the tissues and organs of the fundus elsewhere. The diaphragm is a very definite structure stretching from the cerebral capsule to the body-wall in the region of the nuchal membrane (PI. L
. Contribution to the natural history of the pearly nautilus. Nautilidae. PERISTOMIAL HAEMOCOEL ; SYSTEMIC AORTA; CEPHALIC ARTERIES. 781 side, when a forest of slender conjunctive trabeculae are found passing from the inner (mesial) surface of the liver to the wall of the crop. The peristomial haemocoel is virtually separated from the peri-oesophageal space by a tendinous diaphragm on the dorsal side, and by the tissues and organs of the fundus elsewhere. The diaphragm is a very definite structure stretching from the cerebral capsule to the body-wall in the region of the nuchal membrane (PI. LXXXI. fig. 7, and PL LXXXIII. fig. 26). The nerve-collar which strictly surrounds the oesophagus is the cerebro-visceral loop. The pedal commissure is separated from contiguity with the oesophagus by an interval which is occupied by the ventral retractor muscles of the buccal mass. The oesophagus passes into the buccal mass precisely at the level of the fundus of the peristomial haemocoel, so that the oesophagus proper lies entirely within the peri-oesophageal haemocoel, the buccal mass1 with its attendant muscles and nerves being the only portion of the digestive apparatus which occurs within the limits of the peristomial haemocoel. The cerebral portion of the cerebro-visceral nerve-collar lies within the peristomial haemocoel (PL LXXXI. fig. 7) while the vis- ceral portion lies within the peri-oesophageal haemocoel (PL LXXXIII. fig. 26). Within the mass of the cephalopodium at the angle of junction of the cerebral and visceral por- tions of the nerve-collar occur the cephalic organs of special sense (eye, rhinophore, and otocyst) and their nerves. The demarcation of these components of the nervous system is therefore clear. The greater or systemic aorta of Nautilus arises as a large truncus arteriosus2 from the left dorso-posterior region of the heart and leaves the pericardium through the left peri- cardio-visceral fontanelle. On its mesial side as it leaves the heart it is
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