Anatomy

AnatomyAnatomy:
The science concerned with the shape, structure and the relationship of parts of organisms; also called morphology. is the scientific study of the structure of living things. It includes gross anatomyGross Anatomy:
Anatomy of organs and their arrangements, usually studied without the use of high magnification., the study of anatomical organs and their arrangements (usually without the use of high magnification), and microscopic anatomyMicroscopic Anatomy:
The branch of anatomy in which the structure of cells, tissues, and organs is studied with a light or electron microscope., the study of minute structures with the assistance of a magnifying device (such as a microscope). The latter includes histologyHistology:
The study of cells and tissues at the microscopic level., the microscopic study of the structure of tissues. The study and comparison of similar structures among species is called comparative anatomyComparative Anatomy:
The comparative study of the structure of organisms with regard to homologous organs or parts.. Comparative anatomical studies can provide extensive evidence for evolution and the relationships between species.

Homologous & Analogous Traits

Anatomists especially look for homologousHomologous:
Having the same structure and origin, although current function might differ. traits, that is, those that are similar among species because they were inherited from a common ancestor. For example, the arm of a human, the forelimb of a giraffe, and the wing of a bat are homologous. Even though the wing has a very different function, all three limbs have a very similar bone, muscle, and nerve structure. The limbs were adapted for different needs through natural selection from the limb of the common ancestor of all three of these mammals.

Skeleton of a Flying Fox.

Skeleton of a Giraffe.

Skeleton of a human male, from an engraving.

AnalogousAnalogous:
Having similar function, although different in structure and origin. traits do not look superficially different from homologous traits, but are very different in origin. Such traits have a similar function but evolved independently from different sources (different common ancestors), which we only know by understanding the evolution of a group (through phylogenetic analysisPhylogenetic Analysis:
Analytical method used to find a hypothesis of relationships among species, by coding the various states of homologous characters; also called cladistics. and/or the fossil record). For example, Blue Mussels (family Mytilidae) and Zebra Mussels (family Dreissenidae) are each "mussel shaped" (called mytiliformMytiliform:
Mussel-shaped.) which allows crowding of many individuals into a small space, each attached by elastic byssal threads. However, their phylogenetic tree shows us that the two families are only distantly related, therefore the trait "mussel-shaped shell" is analogous, not homologous. See also convergenceConvergence:
Evolutionary change in two or more unrelated organisms that results in the independent acquisition of similar traits. (discussed below).

Blue Mussels (Mytilus, at left) and Zebra Mussels (Dreissena, at right) are distantly related (far apart on this cladogramCladogram: A tree diagram depicting patterns of shared characteristics and relationships of organisms, generated through phylogenetic analysis.), although both have mytiliform shells, acquired independently due to similar habitat.

We can see clues to analogy (nonhomology) in anatomy — analogous structures are sometimes built of different parts. The wing of a bat and the wing of a bird are analogous — both allow the animal to fly — but are built of very different bones.

Comparative Anatomy

If an unusual-looking oyster is discovered, researchers will most likely conduct an anatomical study to determine whether it is simply an unusual variant of a known species or if it should be considered a new species. If the anatomy of the new oyster is essentially identical to a known species, it could be considered a member of that species. If the anatomy is structurally different, it could be considered a new species. How different the anatomy must be for a variant to be considered a new species is different for different groups of organisms. Of course, other factors, such as habitat and molecular sequences, also contribute to the final decision.

The importance of comparative anatomy in providing evidence for evolutionary history can be seen visually. The images that follow provide a comparative anatomical study of bivalves. In these drawings, the anatomical systems are artificially color coded for easy comparison — digestive systems (bulbous stomach and long intestine) in green, gills in yellow, labial palps (that assist in sorting foot particles) in light blue, foot in brown, muscles and nervous systems in gray, heart in red, byssal threads (used to attach the bivalve to rocks) in dark blue, and siphons and mantle (the shell-producing tissue) in pink (see the Hard-shelled Clam for labels, click to enlarge). The anatomical similarities are strong evidence for common ancestry. The more similar the anatomy, the more closely related the species, and (usually) the less time that they been evolving separately. The anatomical differences are strong evidence for natural selection and adaptation. The more different the anatomy, the less related the species and the more time they have been evolving separately.

Hard-Shelled Clam (Mercenaria)	Blue Mussel (Mytilus)	Soft-Shelled Clam (Mya)
Ark Clam (Arca)	Zebra Mussel (Dreissena)	Dipper Clam (Cuspidaria)
Scallop (Pecten)	Oyster (Crassostrea)	Shipworm (Teredo)

Notice the following:

1. Most of these organs are present in all of the bivalve examples. So the common ancestor of all bivalves likely had all of these organs.
2. Some of the differences can tell us something about life modes and habitat. Some have siphons for living below the surface of the sand or mud. Others have byssal threads (secreted by the foot) for living above the sand, attached to a hard surface such as a rock. Tentacles can only function above the surface of the sand — see these at the tips of the siphons and completely around the entire Scallop (which lives on the surface of the sand).
3. The similarities might be an example of more recent common ancestry: the Oyster and Scallop are members of the subclass Pteriomorphia. They have a single large muscle in the center of the shell instead of one smaller one at each end, plus very large gills, and a very small or completely absent foot; these are examples of similarities due to common ancestry.
4. Other similarities are examples of evolutionary convergenceConvergence:
   Evolutionary change in two or more unrelated organisms that results in the independent acquisition of similar traits.: Blue Mussel and Zebra Mussel are both “mussel shaped” and have byssal threads, but their digestive systems are very different, and the Zebra Mussel has siphons (it does not burrow, but uses the siphons to reach water while packed in very dense colonies).
5. Other traits are unique and indicate adaptation to a particular life mode:
• The very long body, small shell, and extra “caecum” in the digestive tract of the Shipworm — it drills into wood, living protected in a burrow, eating wood particles (assisted by cellulose-digesting bacteria in the caecum), so does not need a large protective shell.
• The tiny gill “windows” of the Dipper Clam — this species is a carnivore, using its long siphon to capture small shrimps, pull them in to the funnel-like mouth, and crunch them in the muscular stomach. It does not use large gills to filter food particles.
• The mantle tentacles, each with an eyespot at its base, of the Scallop — scallops live on the surface of the sea floor, and clap their shells together to swim using a form of jet propulsion. The tentacles and eyespots allow the Scallop to sense its surroundings, most especially potential predators from which it can escape by swimming rapidly away.