312
G. SHELTON
likely that it will fall during the suction pump phase since this is the
time when the gill chamber is expanding in volume. Under these conditions the flow of water during the suction phase would be considerable. The differential pressure curves obtained by Hughes (1960a) suggest that the parabranchial suction pump is at least as important as the
orobranchial pressure pump in dogfish and of greater importance in the
skate. Teichmann (1959), who measured the pressures achieved before
and after functional disturbance of different parts of the system,
claimed that the pressure pump dominated in Scyliorhinus, there was
a balance in Mustelus, and the suction pump was of greater significance
in Torpedo. In this work the time course of the ventilation pressures
was not recorded. The whole relationship between pressure and 00w is
an extremely difficult problem which is still virtually unresolved in
these animals.
The dominance of the parabranchial suction pump in skate and
rays was correlated with the bottom-living habit by Hughes (1960a).
The absence of gradient reversal in the differential pressure curves of
these animals was also associated with their ecology and was thought
to be the result of active closure of the external openings to the gill
slits. This could ensure that no sand entered the gills by reflux through
the external gill openings. Water intake took place through the spiracle
when the animals were partially buried in sand, but both mouth and
spiracles were used if the animals were swimming or resting with the
snout raised. The more pelagic selachians also showed respiratory
adaptations. Some sharks stopped their breathing movements when
swimming actively and ventilatcd their gills simply by opening their
mouths in the water stream. It may be that, as in teleosts of similar
habits, ventilation is controlled by the extent to which the mouth is
opened, but so far this type of regulation has not been confirmed by
observation.
2. THE PUMP MUSCULATURE AND SKELETON
The skeleton of the dogfish head is represented diagrammatically in
Fig. 9. The head muscles of selachians are, with one or two exceptions,
poorly differentiated but those that are defined are shown in Fig. 9,
where the arrows on each muscle give some indication of the effect
produced by contraction. With the exception of the hypobranchials
( coracohyoideus, coracobranchialis ) , the muscles shown are adductor
or adductorlike in function. Their contraction reduces the volume of
orobranchial and parabranchial cavities. In addition to the muscles
G. SHELTON
likely that it will fall during the suction pump phase since this is the
time when the gill chamber is expanding in volume. Under these conditions the flow of water during the suction phase would be considerable. The differential pressure curves obtained by Hughes (1960a) suggest that the parabranchial suction pump is at least as important as the
orobranchial pressure pump in dogfish and of greater importance in the
skate. Teichmann (1959), who measured the pressures achieved before
and after functional disturbance of different parts of the system,
claimed that the pressure pump dominated in Scyliorhinus, there was
a balance in Mustelus, and the suction pump was of greater significance
in Torpedo. In this work the time course of the ventilation pressures
was not recorded. The whole relationship between pressure and 00w is
an extremely difficult problem which is still virtually unresolved in
these animals.
The dominance of the parabranchial suction pump in skate and
rays was correlated with the bottom-living habit by Hughes (1960a).
The absence of gradient reversal in the differential pressure curves of
these animals was also associated with their ecology and was thought
to be the result of active closure of the external openings to the gill
slits. This could ensure that no sand entered the gills by reflux through
the external gill openings. Water intake took place through the spiracle
when the animals were partially buried in sand, but both mouth and
spiracles were used if the animals were swimming or resting with the
snout raised. The more pelagic selachians also showed respiratory
adaptations. Some sharks stopped their breathing movements when
swimming actively and ventilatcd their gills simply by opening their
mouths in the water stream. It may be that, as in teleosts of similar
habits, ventilation is controlled by the extent to which the mouth is
opened, but so far this type of regulation has not been confirmed by
observation.
2. THE PUMP MUSCULATURE AND SKELETON
The skeleton of the dogfish head is represented diagrammatically in
Fig. 9. The head muscles of selachians are, with one or two exceptions,
poorly differentiated but those that are defined are shown in Fig. 9,
where the arrows on each muscle give some indication of the effect
produced by contraction. With the exception of the hypobranchials
( coracohyoideus, coracobranchialis ) , the muscles shown are adductor
or adductorlike in function. Their contraction reduces the volume of
orobranchial and parabranchial cavities. In addition to the muscles