Shell pictures, select a living family below:
Classification (Smithsonian Institution, 4 May 2001;
modified):
- Subclass: Nautiloidea (Tetrabranchiata)
- Order: Nautilida
- Superfamily: Nautiloidea
- Family: Nautilidae
- Order: Orthocerida (extinct)
- Subclass: Ammonoidea (ammonites, extinct)
- Subclass: Coleoidea (Dibranchiata)
- Order: Belemnoidea (extinct)
- Order: Spirulida
- Family: Spirulidae
- Order: Sepiida (cuttlefish)
- Family: Sepiadariidae
- Family: Sepiidae
- (others)
- Order: Sepiolida (cuttlefish)
- Family: Idiosepiidae
- Family: Sepiolidae
- (others)
- Order: Teuthida (squids)
- Suborder: Myopsina
- Family: Loliginidae
- Suborder: Oegopsina
- Family: Onychoteuthidae
- Family: Architeuthidae
- Family: Ommatostrephidae
- Family: Sthenoteuthidae
- (others)
- Order: Vampyromorphida (vampire squid)
- Family: Vampyroteuthidae
- Order: Octopoda
- Suborder: Cirrina (finned octopus)
- Family: Cirroteuthidae
- Family: Stauroteuthidae
- Family: Opisthoteuthidae
- Suborder: Incirrina (octopus)
- Family: Bolitaenidae
- Family: Octopodidae (many species)
- Family: Argonautidae
- (others)
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Overview
The class, Cephalopoda, includes the Chambered Nautilus, cuttlefish, squid, and
octopus, as well as fossil ammonite and other fossil forms. It is a very diverse class,
with 600 living species and more than 7500 fossil species. In the evolution of cephalopods, three common
fossil groups, the Orthocerida (Orthoceras species), Ammonoidea, and
Belemnoidea are particularly interesting. Although not forming a monophyletic line, they
indicate progressive evolutionary steps from slow moving shelled forms to high-speed
shell-less forms like present-day squid and octopus.
Size varies from planktonic to some as large as sixty feet; e.g., the present day squid, Architeuthis
princeps, found in open ocean. However, most cephalopods are small, and they
form a major component of the food web of larger fish and whales, as they have for at
least the past 200 million years. Both abyssal and shallow water forms are found.
Anatomically, the tetrabranchiate body plan is seen only in the Chambered Nautilus;
whereas a dibranchiate body plan is characteristic of all other living cephalopods.
"Tetrabranchiate," refers to a duplication of the paired visceral organs found
in the more primitive living molluscs. The Chambered Nautilus seems to be more evolutionarily primitive
in other respects also, e.g., simpler eye structure, numerous undifferentiated tentacles;
and, it is rather sluggishly responsive. For these reasons, it is sometimes referred to as a
surviving fossil species.
Squids ("decapods") are dibranchiate. They have eight arms and
two tentacles, body fins, and an internal shell vestige for muscle attachment,
reminiscent of the extinct belemnites. What remains in squid, is either
a small, horny plate or rod completely encased by the mantle. Paired tentacles
are used for lightning-quick grasping of prey, while the other tentacles
are used for holding. Cuttlefish, in comparison to squids, have a wholly
calcified internal shell, which is their distinguishing characteristic.
In all the dibranchiates, either an interior vestigial shell or cartilaginous
collar surrounds the highly organized brain ganglia.
Octopus species also are dibranchiate. They have eight equally long arms, and no shell
or internal vestige of a shell. Most have no fins (Suborder: Incirrina). The
Vampyroteuthidae (Vampire "Octopus") are actually small squids, but the two
small tentacles are not obvious. In the Spirulidae family, Spirula spirula
is exceptional in having a coiled and partly calcareous shell, and it is otherwise similar
to a squid.
Dibranchiates have exceptionally well developed sense organs, including those for
discriminating touch, chemoreception (taste), equilibrium, and excellent sight. Some deep
water squid can also produce stroboscopic, bioluminescent beams of light, probably used to
stun or confuse prey. Numerous accounts exist of intelligent, learned behavior in
octopods, both in the anecdotes of SCUBA divers, and in documentation by time lapse
photography.
Squid are most adept at swimming. They may swim either by using their fins, which are
outgrowths of the mantle, or by the more energetic means of jet propulsion, like the
octopus. Accounts are available of Onychoteuthis flying out of the water by
jet propulsion, becoming airborne, then gliding by means of its outspread tail fins (Morton, 1960). Other squid notable for taking to
the air are the Ommatostrephidae and Sthenoteuthidae.
Octopods are particularly expert at camouflage and hiding, although these traits are
shared to a fair extent also with squid. A system of contractile cells (chromatophores) is
found in the skin, each containing any one of three or four pigments. The
chromatophores can be turned on or off immediately, to produce startling color changes.
Accounts exist, for example, of a deep purple octopus discharging its ink sac to blot
out visibility, blanching its skin, then quickly emerging behind the ink blot as a
practically invisible animal.
OTHER LINKS -THE CEPHALOPOD PAGE &
CEPHBASE
by Dr. James B. Wood
(live cephalopods)
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