PRINCIPLES AND EMERGING PATTERNS - CHAPTER 2
All animals must solve the same problems of existence-procurement of food and oxygen,
maintenance of salt and water balance, removal of metabolic wastes, and perpetuation of the
species. The body design necessary to meet these problems is, in large part, correlated with four
factors: (1) the type of environment-marine, freshwater, or terrestrial; (2) the size of the animal; (3)
the mode of existence; and (4) the constraints of the animal's genome. The last factor refers to
limitations imposed by the ancestral design by the animal's genetic makeup.
Effects of Environment
Marine Environment - The marine environment is generally the most stable. Wave action,
tides, and vertical and horizontal ocean currents produce a continual mixing of sea water, reducing
fluctuations in the concentration of dissolved gases and salts. Seawater is buoyant, reducing
problems of support (most of the large invertebrates are marine). Seawater is more or less
isoosmotic to body tissue fluids, meaning it is easy to maintain salt and water balance. The
buoyancy and uniformity of seawater also provide an ideal medium for reproduction. Eggs can be
released into the water where they can be fertilized and go through embryonic development with
little danger of dessication, salt imbalance, or of being swept away into a harsh environment.
Marine invertebrates have the greatest diversity of larval forms, an highly advantageous condition.
NOTE: intertidal less stable.
Freshwater Environments - less stable than ocean. Streams vary greatly in turbidity,
velocity, volume, and chemically on both a spatial and temporal scale. Ponds and lakes fluctuate in
oxygen content, turbidity, chemistry, and water volume.
Freshwater still provides support, although not all well as seawater.
The low salt
concentration creates some difficulty in maintaining water and salt balance.
Freshwater is
hypotonic to body tissue, meaning that freshwater invertebrates must have some mechanism for
pumping water out of the body while retaining salts (osmoregulation).
In general, eggs are either retained by the parent or attached to the bottom. This reduces the
likelihood that the eggs will be transported into a harsh environment. Larval stages are greatly
reduced or absent. When they are present, as in holometabolous insects, the larval stage is capable
of locomotion and does not drift in the water column.
Nitrogenous wastes of aquatic animals, both freshwater and marine, are usually excreted as
ammonia. Ammonia is very soluble but is also toxic and requires a large volume of water be
excreted with it (this helps get rid of excess water).
Terrestrial Environments - live in the harshest environment. The support of water is
absent. Most critical is the loss of water by evaporation. The primary solution to this problem has
been the evolution of an integument that minimizes or eliminates water loss across soft tissues.
There is more oxygen present on land than in water; however, respiratory structures must remain
moist and are usually housed internally to prevent water loss.
Many of the physiological,
morphological, and behavioral adaptations seen in invertebrates are to minimize water loss.
Nitrogenous wastes are commonly excreted as urea or uric acid which are less toxic than
ammonia and require less water for removal. With the exception of insects and some arthropods,
development is direct. Internal fertilization is a must and eggs must be held internally or placed in a
moist habitat so as not to dry out.
Effects of Body Size
As body size increases, volume (a cubed measure) increases faster than surface area (a
squared measure). This has been very important in the evolution of invertebrates as it effects the
degree of interaction between organism and environment as well as how the organism will function
physiologically. In small animals, the surface/volume ratio is high and allows for external and
internal transport to occur by diffusion. As a result, specialized systems for the acquisition of
oxygen and food or the excretion of waste may not be necessary. In large animals, the distance
from surface to interior increases, meaning simple diffusion is not enough. As you will see, their is
a direct relationship between increasing complexity of organ systems and the ability to achieve large
body size in animals.
Another common adaptation related to body size is the folding or coiling of tissues. This
simply achieves greater surface area in a smaller space.
Effects of Mode of Existence
The structural features of an organism are going to dictate how it lives. Free-moving
animals are generally bilaterally symmetrical. The sensory organs and nervous system are usually
concentrated at the anterior end since this is the area that will first make contact with any stimulus.
Attached or sessile animals are usually radially symmetrical or tend toward a radial symmetry in
which the entire body or some part of the body consists of a central axis around which similar parts
are symmetrically arranged. This allows the animal to read stimulus from all sides, a factor that is
very important if you cannot move to find and locate the source of the stimulus.
Cellular Specialization
We are not going to discuss this. Most of the information should have been covered in Cell
Biology. Are still responsible for this section.