INTRODUCTION

 

 

Introduction 

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The Cephalopoda is an ancient and very successful group of the Mollusca. Cephalopods have been among the dominant large predators in the ocean at various times in geological history. Two groups of cephalopods exist today: The Nautiloidea with a few species of the pearly nautilus, and the Coleoidea, containing the squids, cuttlefishes, octopods and vampire squids, which is represented by about 700 species. Cephalopods are the most active of the molluscs and some squids rival fishes in their swimming speed. Although there are relatively few species of living cephalopods, they occupy a great variety of habitats in all of the world's oceans. Individual species are often very abundant and provide major targets for marine fisheries.

Cephalopods first appeared about 500 million years ago in the Upper Cambrian Period. Although considerable uncertainity still exists, the two extant lineages may have separated 470 mya with the possible origin of the Bactritida or earlier. The long separation of the two lineages has, today, resulted in lineages with cephalopods that are very different in structure.

Characteristics

  1. A funnel derived from the molluscan foot.
  2. Circumoral arms that are probably derived from the molluscan head.
  3. Chitinous beaks.
    The photograph on the right shows the buccal mass of the giant squid Architeuthis. The dark, chitinous, bird-like beaks are visible on the surface of the white muscle mass.
  4. Complex chambered shell.
    The shell bears a phragmocone and siphuncle (lost in most Recent cephalopods). On the left is the shell of the pearly nautilus cut in half along the midline to show the flotation chambers (phragmocone) and the siphuncle (lost from some chambers) that passes through them.
  5. Image-forming eyes.

Classification

Much of the higher classification of Recent cephalopods is unstable. Various authors have suggested highly varying arrangements. We adopt a conservative arrangement that does not differ much from that of Naef (1921-23). Except for the position of the Octopodiformes and its two orders, we have questionable phylogenetic basis for accepting this or any other scheme. We suggest, however, for the sake of stability, that the following classification be used until this or an alternative arrangement can be derived from cladistic analyses. The analyses, whether molecular or morphological, however, must be robust and must resist considerable scrutiny before changes in classification should be adopted.


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