MOLLUSKS 159 
bers and the whole interior of the shell are lined with pearly nacre, 
having a lustre only slighty inferior to that of the pearl oyster. 
The nautilus is one of the most interesting of living animals, 
for it is the sole survivor of a once numerous race, great numbers 
of which swam in the ocean during Silurian times. The oldest 
forms are apt to have straight shells, but later we find them for the 
most part coiled, while during the last ages of their decline they 
sometimes uncoiled to a greater or less degree, or assumed strange 
contorted shapes. Some were five or six feet in length, and one 
species must have weighed several tons. 
Associated with the nautilus race was another great group 
called the Ammonites in which the margins of the partitions separ- 
ating the chambers of the shell were complexly folded. Their shells 
were also highly ornamented with ridges and projections and the 
siphuncle was small and did not usually pass through the centre 
of the partitions, but ran through the edges close to the wall of the 
shell. More than 5000 species of Ammonites and 2500 of the nau- 
tilus race lived in these ancient times. The Ammonites died out 
completely in the age of the chalk, while the nautilus race declined 
slowly, until to-day we find its last representatives still living in 
the depths of the tropical Pacific. Three or four species of Nauti- 
lus are found in the tropical Pacific, the best known being Nautilus 
pompilius. 
THE SQUIDS, OR SEA-ARROWS 
Several species of squids occur along our coast. Their bodies 
are spindle-shaped, tapering to a point behind, while the fin resem- 
bles in outline an arrow or spear-head. The shell is degenerate, 
and is reduced to a mere internal scale imbedded in the mantle and 
called the “pen” in allusion to its shape. Ten arms surround the 
mouth. Eight of these are triangular in cross section, and are each 
furnished with two rows of suckers on their inner sides. The 
fourth pair of arms are, however, much longer than the others, 
and have suckers only upon their expanded tips where we find four 
rows of these organs of adhesion. ‘The eyes are large, have no lids, 
and the pupil is a round opening. 
Squids usually swim backw ard, being propelled in a series of 
rapid darts by the water which is sucked in through the mantle- 
