MOLLUSCA. 267 
very abundant: more than 2000 fossil species are known, 
but only one living representative—the Pearly Nautilus. 
1. Tetrabranchs.— This order is characterized by the 
possession of four gills, forty or more short tentacles, and 
an external, chambered shell. The partitions, or septa, of 
the shell are united by a tube called “siphuncle,” and the 
animal lives in the last and largest chamber."* The living 
Nautilus has a smooth, pearly shell, a head retractile with- 
in the mantle or “hood,” and calcareous mandibles well 
fitted for masticating Crabs, on which it feeds. This strag- 
Wr 
¥1a. 238.—Pearly Nautilus, with shell bisected; one-half natural size. Indian Ocean. 
gler of a mighty race dwells in the deep parts of the 
Indian Ocean, crawling on the bottom; and while the 
shell is well known, only two or three specimens of the 
animal have ever been obtained. 
2. Dibranchs.—These are the most active of Mollusks, 
and the tyrants of the lower tribes. Among them are 
the largest of invertebrate animals. They are naked, 
having no external shell covering the body, but usual- 
ly a horny or calcareous part within. They have a 
distinct head, prominent eyes, horny mandibles, eight or 
ten arms furnished with suckers, two gills, a com- 
