Class Cephalopoda

The cephalopods are the most highly organized of the mollusks, with the greatest complexity of any of the spiralian groups. The close association of a well-defined head with the foot modified into tentacles is the source of their name, meaning "head-footed". High metabolic and mobility rates, a well-developed nervous system, and sharp eyesight associated with an advanced brain, are ideal adaptations for a carnivorous predatory life mode. The funnel or hyponome is also modified from the foot, and squirts out water from the mantle cavity providing the animal with a form of jet propulsion.

Modern cephalopods belong to two groups. Firstly, living Nautilus has an external coiled shell with a thin internal mantle and nearly 100 tentacles. Only five species of this genus are extant although it was once used as an analog for the behavior of all extinct externally-shelled cephalopods such as the ammo-noids. Secondly, the coleoids; these have internal shells and thick external mantles. They include the 10-tentacled extinct belem-nites, the squids and cuttlefish; the octopods have eight tentacles and have lost their skeleton. These living forms are most common in shallow-water belts around the ocean margins.

A tripartite division of the cephalopods into three subclasses includes: (i) Nautiloidea, with straight or coiled external shells with simple sutures (Late Cambrian to Recent); (ii) Ammonoidea, with coiled, commonly ribbed external shells with complex sutures (Early Devonian to latest Cretaceous, possibly earliest Paleogene); and (iii) Coleoidea, with straight or coiled internal skeletons (Carboniferous to Recent).

The origin of the cephalopods remains controversial, although most agree the group was derived from a monoplacophoran-like ancestor. John Peel (1991) suggested that the group is derived from within the class Helcionel-loida; both groups are characterized by endo-gastric coiling and, moreover, the helcionelloids predate the appearance of the cephalopods by some 10 million years. Another group of gastropod-like shells, the tergomyans, with apical septa, might also have been ancestral, only they lack perforate septa.

Nautiloidea_

Most information about nautiloids comes from studies of the behavior and morphology of the living Nautilus that occurs mainly in the southwest Pacific, normally at depths of 5-550 m (Box 13.6). It pursues a nocturnal, nektobenthonic life mode as both a carnivore and scavenger; however it is prey to animals with powerful jaws such as the perch, marine turtles and sperm whales.

Living Nautilus has its head, tentacles, foot and hyponome concentrated near the aperture of the body chamber; the visceral mass containing other vital organs is situated to the rear of the body chamber (Fig. 13.14). The surrounding mantle extends posteriorly as the siphuncular cord connecting all the previous, now empty, chambers that together constitute the phragmocone. Each chamber is partitioned from those adjacent by a sheet of calcareous material, the septum; the suture is formed where each septum is cemented to the outer shell. The form of the suture, or the suture pattern, is used in the classification of externally-shelled cephalopods. The conch is usually oriented as follows: anterior at the aperture, posterior at the point furthest from the aperture, the venter on the side with the hyponome, usually the outside, and the dorsum opposite. Despite the simplicity of this arrangement, fossil nautiloids developed a wide range of shell morphologies (Fig. 13.15).

Ammonoidea

The ammonite usually had a planispirally coiled shell comprising the protoconch, phrag-mocone and body chamber (Fig. 13.16). The protoconch or larval shell records the earliest siphuncle mural part of septum septal line dorsal nacreous layer

color bands siphuncular cord color bands siphuncular cord siphuncle mural part of septum septal line

Siphuncular

orthoconic cyrtoconic orthoconic cyrtoconic lituiticone longicones brevicones

gyrocone

Figure 13.14 (a) Features of the shell and (b) internal morphology of a living Nautilus. (c) Shell shapes of the nautiloids.

orthoconic cyrtoconic orthoconic cyrtoconic lituiticone longicones brevicones

gyrocone

Figure 13.14 (a) Features of the shell and (b) internal morphology of a living Nautilus. (c) Shell shapes of the nautiloids.

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