What Starfish Dissection Reveals
As is the case with most dissection procedures, the approach taken is not one of dissecting the subject to see what is to be found, but to learn something of the anatomy of the subject ahead of time. In the case of a starfish, one will then start by gaining an understanding of both its external anatomy and its internal anatomy.
The external anatomy of a starfish consists of the central disc and the rays or arms, which of course may vary in number, but most starfish species have 5, the aboral surface ( the top surface) featuring dermal branchiae (the skin gills) and pedicellarae, the mouth, water vascular system and tube feet, and for most species, ambulacral spines. The skin gills can be seen by close examination and resemble tiny bumps on the skin. These bumps absorb oxygen from the sea water. The other bumps, the pedicellarae, are actually small organs which constantly clean the surface of the skin. If this did not happen, barnacles would likely find a home on the upper surface of the starfish. The external anatomy of this creature also contains what is known as a sieve plate. An understanding of how the starfish lives, moves around, and eats, as well as its habitat will give the student performing the starfish dissection a greater appreciation of the function of these external anatomical features.
In most laboratory settings, a student will need to identify the major external anatomical characteristics before beginning the actual dissecting. The "legs" for example, are more accurately referred to as arms or rays. The starfish moves not so much by moving the rays, but by moving its tube feet. The rays follow the direction the tube feet takes them. The tube feet will have suckers which are used both to aid in locomotion, hold on to rocks, and capture and hold prey. There may be several hundred tiny tube feet beneath each ray. These feet are filled with water which is supplied by the vascular. The vascular system pumps water into the tube feet, causing them to expand, allowing each individual foot to move. Some of us have trouble coordinating movement of two feet at times, while the starfish seems to manage quite well in moving hundreds of them in the right direction. The ambulacral spines also aid in locomotion.
When the dissecting process is underway, and the internal anatomy begins to reveal itself the starfish begins to appear a little less weird and a little more animal or fish-like. It should be noted here that the starfish is not actually a real fish, in that it lacks fins and vertebrae. Internally, one will discover, the stomach, intestines, digestive glands, the reproductive organs, and various canals and pouches. Before actually dissecting a starfish it's usually advisable to wear a lab apron and goggles, as the starfish does not give itself up willingly. It can be tough to cut, even with a very sharp scalpel, and unlike other animals, has a tendency to send out flying pieces of debris as dissecting cuts are made.
Starfish Dissection - The first cut is usually the tip of one of the rays, followed by two long incisions along the ray so the skin may be peeled back. This will expose the digestive glands, found in each ray, and the canals connecting all of the rays to the central disc. The skin is then cut away from the central disk, which will bring the stomach into view. The reproductive organs will also be found in the same general area as the stomach. Locating these organs won't tell much about the sex of the specimen, as male and female organs are virtually identical, at least on the surface.
From one point of view, as interesting as the internal anatomy of a starfish may be, and starfish dissection is the only way to discover and understand the inner workings, the outside of the starfish, with its rays, spines, skin, mouth, and feet, is every bit as interesting. The biology lab however will require correct documenting or labeling of the principal parts, both externally and internally, to give a passing grade.