Animals without independent vascular systems
A sphere represents the smallest possible ratio of
surface area to volume; modifications in architecture,
reduction of metabolic rate, or both may be exploited
to allow size increase. Sponges overcome the problem
of oxygen supply and increase the chance of food capture
by passing water through their many pores using ciliary
action. The level of organization of sponges is that
of a coordinated aggregation of largely independent
cells with poorly defined tissues and no organ systems.
The whole animal has a relatively massive surface area
for gaseous exchange, and all cells are in direct contact
with the passing water current.
Among the eumetazoan (multicellular) animals the cnidarians
(sea anemones, corals, and jellyfish) are diploblastic,
the inner endoderm and outer ectoderm being separated
by an acellular mesoglea. Sea anemones and corals may
also grow to considerable size and exhibit complex external
structure that, again, has the effect of increasing
surface area. Their fundamentally simple structure—with
a gastrovascular cavity continuous with the external
environmental water—allows both the endodermal and ectodermal
cells of the body wall access to aerated water, permitting
direct diffusion.
This arrangement is found in a number of other invertebrates,
such as Ctenophora (comb jellies), and is exploited
further by jellyfish, which also show a rudimentary
internal circulatory system. The thick, largely acellular,
gelatinous bell of a large jellyfish may attain adiameter
of 40 centimetres (16 inches) or more. The gastrovascular
cavity is modified to forma series of water-filled canals
that ramify through the bell and extend from the central
gastricpouches to a circular canal that follows the
periphery of the umbrella. Ciliary activity within the
canals slowly passes food particles and water, taken
in through the mouth, from the gastric pouches (where
digestion is initiated) to other parts of the body.
Ciliary activity is a relatively inefficient means of
translocating fluids, and it may take up to half an
hour to complete a circulatory cycle through even a
small species. To compensate for the inefficiency of
the circulation, the metabolic rate of the jellyfish
is low, and organic matter makes up only a small proportion
of the total body constituents. The central mass of
the umbrella may be a considerable distance from either
the exumbrella surface or the canal system, and, while
it contains some wandering amoeboid cells, its largely
acellular nature means that its metabolic requirements
are small.
