(1) The rates of oxygen consumption of Thais
lnpillur. hittorina littorea, L. obtueata. h, saxatilis
and possibly of neritoides in air, and when immersed
in water, give a good indication of their reactions to
various physical and chemical conditions which may be
imposed experimentally, and which are designed to simulate conditions which these species may encounter on the
(2) Gastropods from higher tidal levels tend to
show a progressive ability to utilise atmospheric oxy¬
gen, but the rates of respiration of each species either
in air or in water display great variability. Some of
these respiratory luctuatione may be attributable to
the external influences of the time of day, and the
effect of tidal rhythm, to which the animal reacts.
(3) Factors of primary importance in determining
the vertical distribution of the five species of intertidal
gastropod under investigation in this thesis, are
considered to be the combined effects of the temperature
of asphyxiation in air, together with the ability of
the animal to withstand desiccation. Above the air
asphyxiating temperature an individual becomes immobilised,
the operculum remains ope,±, and if desiccation cannot
be prevented, the animal may die owing to the latter
cause. There is a direct correlation between these
two effects, and the vertical distribution of Thais and
the hittorina spp. on the shore; obtusata is particularly susceptible tb desiccation, and this further
confines the species to a moist, weedy habitat.
(4) The effects of the teiaperature of asphyxiation
in water and the temperature which is lethal to a species,
are of secondary importance. An animal immersed in
water will become immobilised above the temperature of
asphyxiation, but it is not exposed to any other harmful factor, and when the temperature falls below this
critical point the animal can resume normal activity.
The lethal temperature is too high to be of practical
account to an animal under anything but abnormal
(5) There is some evidence that the rate of water
loss is correlated with the tidal level inhabited by a
species on the shore, but the behaviour of the species
on being subjected to conditions of desiccation is a
factor which is closely linked with water loss. On
exposure to dessication, the death of an animal is thought
to be caused by suffocation due to mucus drying on the
respiratory surfaces, rather than by the actual loss of
water in the soft tissues.
(6) From its respiratory behaviour, oaxatilis
indicates that it is less affected by salinity changes
than are the other species. But resistance to variations in salinity is considered to be of secondary importance as a factor determining the vertical distribution of these littoral gastropods.
(7) Amongst the biological factors, the reproductive period and the type of larva produced are most
likely to be effective in determining the distribution
of species; for example in the special distribution of
L. neritoidee. The availability of food, and type of
diet may have a certain influence on habitat, and in
special cases, e.g. T, laplllue, on tidal level.
Tropisms and shape may influence habitat, but are not
considered to be effective in determining the vertical
distribution of a species on the shore.