262
MALCOLM R. CLAFCKE
XXV. STRUCTURAL VARIATION
Oceanic squids vary from rapacious, fast-swimming predators,
like the ommastrephids and onychoteuthids, to passive, balloon-like
drifters like the cranchiids. The rapid swimmers have pointed, muscular
bodies which, by powerful contractions, send jets of water from a
muscular funnel. The efficiency of such a method of locomotion must
be greatly increased by the fact that the thrust-producing contraction
simultaneously reduces the cross-sectional area of the animal and so
reduces drag. The great mobility of the funnel enables the squid to
dart forwards after prey and the fins, far from being mere stabilizers,
can contribute to either forward or backward movement by powerful
beats, the direction of which may be modified by a flexing of the body
in front of the fins. The most active species are slightly negatively
buoyant and need to swim to avoid sinking deeper. Water currents
used for locomotion pass over and oxygenate the gills in the mantle
cavity.
At the other extreme, the cranchiids increase their buoyancy by
retaining ammonium ions in their greatly expanded body cavities
(Denton et al., 1958; Denton, 1960a). Coupled with the less-active
life which such a buoyancy system permits, cranchiids have developed
a method of drawing water over the gills, independent of the locomotary
system (Clarke, 1962~). A horizontal membrane divides the mantle
cavity into a chamber on each side of the coelom and a ventral chamber
(Fig. 59). Water can enter the paired dorsal chambers through the
inhalent openings a t the sides of the head, pass through the paired
spiracles in the horizontal membrane just in front of the gills, into the
single ventral chamber. The water is first drawn in by the contraction
of the wall of the coelom near the anterior end. The contraction
travels posteriorly and pushes before it a " bolus )' of water in each
of the dorsal paired chambers. Upon reaching the spiracle in the
horizontal membrane the '' bolus " passes through into the ventral
chamber from whence it passes out through the funnel. Continual
peristaltic contractions of the coelom wall maintain this respiratory
circulation while occasional contractions of the mantle are used for
escape reactions; otherwise, slow locomotion is carried out by rapid
paddling of the fins.
Between these two extremes of life there are forms such as Taningia
danae and Chiroteuthis veranyi which use locomotory currents to
oxygenate the gills but are very gelatinous and are possibly neutrally
buoyant as a result as found in other animals (Denton and Shaw, 1961).
Structures used for catching prey are very variable. The tentacles
and/or arms may have chitinous rings with or without teeth or these
MALCOLM R. CLAFCKE
XXV. STRUCTURAL VARIATION
Oceanic squids vary from rapacious, fast-swimming predators,
like the ommastrephids and onychoteuthids, to passive, balloon-like
drifters like the cranchiids. The rapid swimmers have pointed, muscular
bodies which, by powerful contractions, send jets of water from a
muscular funnel. The efficiency of such a method of locomotion must
be greatly increased by the fact that the thrust-producing contraction
simultaneously reduces the cross-sectional area of the animal and so
reduces drag. The great mobility of the funnel enables the squid to
dart forwards after prey and the fins, far from being mere stabilizers,
can contribute to either forward or backward movement by powerful
beats, the direction of which may be modified by a flexing of the body
in front of the fins. The most active species are slightly negatively
buoyant and need to swim to avoid sinking deeper. Water currents
used for locomotion pass over and oxygenate the gills in the mantle
cavity.
At the other extreme, the cranchiids increase their buoyancy by
retaining ammonium ions in their greatly expanded body cavities
(Denton et al., 1958; Denton, 1960a). Coupled with the less-active
life which such a buoyancy system permits, cranchiids have developed
a method of drawing water over the gills, independent of the locomotary
system (Clarke, 1962~). A horizontal membrane divides the mantle
cavity into a chamber on each side of the coelom and a ventral chamber
(Fig. 59). Water can enter the paired dorsal chambers through the
inhalent openings a t the sides of the head, pass through the paired
spiracles in the horizontal membrane just in front of the gills, into the
single ventral chamber. The water is first drawn in by the contraction
of the wall of the coelom near the anterior end. The contraction
travels posteriorly and pushes before it a " bolus )' of water in each
of the dorsal paired chambers. Upon reaching the spiracle in the
horizontal membrane the '' bolus " passes through into the ventral
chamber from whence it passes out through the funnel. Continual
peristaltic contractions of the coelom wall maintain this respiratory
circulation while occasional contractions of the mantle are used for
escape reactions; otherwise, slow locomotion is carried out by rapid
paddling of the fins.
Between these two extremes of life there are forms such as Taningia
danae and Chiroteuthis veranyi which use locomotory currents to
oxygenate the gills but are very gelatinous and are possibly neutrally
buoyant as a result as found in other animals (Denton and Shaw, 1961).
Structures used for catching prey are very variable. The tentacles
and/or arms may have chitinous rings with or without teeth or these