. Elements of zoology, or, Natural history of animals / ed. by Reese. Zoology. 448 ZOOLOGY. 776. The head of the nautilus supports a large fleshy disk, upon which it is believed that the animal crawls upon the bot- tom or sides of the ocean, as a snail upon its foot. The animal frequents deep waters ; and though it is occasionally found at the surface, it sinks upon the least alarm, so that it has been very rarely captured, although the empty shell is so common. This power of rising and falling in the water appears in some degree connected with the structure of the shell, and with the mod
. Elements of zoology, or, Natural history of animals / ed. by Reese. Zoology. 448 ZOOLOGY. 776. The head of the nautilus supports a large fleshy disk, upon which it is believed that the animal crawls upon the bot- tom or sides of the ocean, as a snail upon its foot. The animal frequents deep waters ; and though it is occasionally found at the surface, it sinks upon the least alarm, so that it has been very rarely captured, although the empty shell is so common. This power of rising and falling in the water appears in some degree connected with the structure of the shell, and with the mode in which its chambers commu- nicate with the body by a membranous tube which passes through all the partitions, and is called the siphuncle. But of the mode in which this operates, no very satisfactory account has Section of Nantil- yet been given.* us Pompilius. 777. A large number of fossil shells analogous to that of the nautilus, and hence called nautilites, are found in various strata, from the oldest limestones and sandstones of the Silurian sys- tem to those covering the chalk. The fossilized mandibles, termed rhynclwlites, which were at first supposed to have be- longed to birds, are also abundant in some strata, especially in the lias at Lyme CLASS XV.—PTEROPODA. 778. The Pteropoda are a small but remarkable class of Mollusca, distinguished by the adaptation of their structure for active locomotion in water only. Their general organization is higher than that of the Gasteropods; and they are particu- larly distinguished by the possession of a pair of fin-like organs, which are attached to the anterior part of the body, and are evi- dently adapted for propelling it rapidly through the water. The body is uniformly symmetrical; that is, its two sides precisely correspond—a condition obviously favourable for rapid move- ment. It is from the wing-like character of these lateral ap- pendages that the name of the class is derived ; and the animals composing it m
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