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 pcean 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 Ammunites 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. Avhile 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 backward, being propelled in a series of 

 rapid darts by the water which is sucked in through the mantle- 



