INVERTEBRATE ZOOLOGY IN THREE BOOKS - BOOK 3 - H.I. Shcherbak - 1997

PHYLUM MOLLUSCA

CLASS CEPHALOPODA

SUBCLASS NAUTILOIDEA

Nautiloids represent the most primitive and ancient group of cephalopods, characterized by an external multichambered shell, two pairs of gills, four atria, four Kidneys, and A large number of arms. The Nervous system is non-ganglionated, forming nerve cords, and the eyes are of a very primitive Structure.

In the modern cephalopod fauna, this subclass is represented by only a single order, Nautilida, comprising several species, although numerous orders and species of extinct nautiloids are known (see below).

Order Nautilida. This order includes the single extant genus Nautilus with six species. The best-known is Nautilus pompilius (see Fig. 90, a). Nautiluses inhabit warm seas near the coasts of the Philippine Islands, New Guinea, Australia, Indonesia, and the Bay of Bengal.

Their shell is 20–25 cm in diameter, spirally coiled, and divided by transverse septa into a series of chambers. The animal's body occupies the last and largest chamber, while the remaining chambers are filled with gas and partially with Water, serving as a hydrostatic apparatus for the mollusk (see p. 137). The outer surface of the shell is white with reddish transverse stripes, and the interior is lined with nacre (mother-of-pearl). The large HEAD is covered dorsally by a massive fleshy hood, which closes the shell aperture when the animal is threatened.

Around the Mouth are numerous (about 90) arms arranged in two whorls—an outer and an inner. They lack suckers, and their distal ends can be retracted into expanded basal portions. The funnel consists of two lobes rolled into a tube. The mouth is equipped with a pair of horny jaws capable of crushing hard food, and a radula is present in the Pharynx. The mantle cavity contains a pair of osphradia. An ink gland is absent.

Nautiluses are slow-moving animals; they inhabit primarily the water Column, descending to depths of 500–700 m, though they may also ascend to surface layers. They hunt at night for small crustaceans and other sluggish prey, but also readily feed on carrion. For reproduction, they migrate to the shallow waters of coral islands, where females lay very large (up to 4 cm) eggs, attaching them singly to submerged objects. The eggs hatch into miniature nautiluses with a well-developed shell that already possesses several chambers. The shell grows along its free margin by forming new chambers, into which the body of the mollusk progressively moves.