Dissection of a Sea Star

Photos by Dr. J. WhaleyFlagler Palm Coast High School 2013

Adult Sea Stars have penta-radial symmetry; therefore, each ray or arm contains thesame organ structures as the other arms. The structures in the central disc are not in multiples of five.

External Anatomy – Aboral Surface (Dorsal Surface)

Madreporite (or seive plate)

Central Disc- at the center the the disc is the anus

Arm or Ray

The larger, longer projections are the spines, which aid in protection.The smaller projections surrounding the spines are the skin gills which function to take in oxygen and

the pedicellariae (small pincher-like structures that help keep the skin clear of debris.)

External Anatomy – Aboral Surface

MadreporiteLocated on the central disc at the junctionbetween two of the arms. Functions to draw water into the water vascular system of the sea star.

Spine

Pedicellariae

Skin Gills

External Anatomy – Oral Surface (Ventral Surface)

Mouth

Tube Feet

Ambulacral Groove

External Anatomy – Oral Surface

Note the rows of tube feet in the ambulacral groove. The tube feet function in locomotion, feeding,respiration and excretion.

Internal Anatomy

After careful removal of the dorsal surface (aboral surface) of skin from one of the arms of a sea star, you may notice the network of structures located directly under the skin. These are the ossicles, that help make up the endoskeleton. They are closely associated with the spines.

Ossicles

Located under the skin and endoskeleton in the arms of a sea star are the digestive glands. They produce digestive enzymes which will help breakdown food substances. These glands appearto take up most of the arm of the sea star. Note the vessels which connect the digestive glands to the stomach region.

Digestive GlandsVessels leading to the stomach

Internal Anatomy

Stomach

To see the stomach, the skin over the central disc has to be carefully removed. When the skin is removed, the connection to the anus is also cut. The dorsal portion of this region is the pyloric stomach. It is hard to distinguish from the cardiac stomach which lies below it.

Internal Anatomy

When the stomach is removed, it should be examined for the presence of any food substances.

In some specimens, the cardiac stomach may be partially everted through the mouth. Remember, the sea star will evert its stomach when eating.

Stomach(as seen from oral surface)

Internal Anatomy

When the digestive glands are moved to the side, the paired gonads are visable . They are located in the arms, close to the central disc. You cannot visually distinguish between the ovaries and testes.

Gonads, paired

Digestive Glands

Internal Anatomy – Water Vascular System Water enters the Madreporite on the aboral surface. From there, water travesl down the Stone Canal to the Ring Canal. Water then enters each arm through theRadial Canals. Lateral Canals (not visible) connect theRadial Canals to the Ampullae. The Ampullae are bulb-like structures which connect to the tops of the tube feet.

Madreporite

Stone Canal - cut

Radial Canal

Internal Anatomy – Water Vascular System

Ring Canal

Radial Canals

Internal Anatomy – Water Vascular System

Radial Canal

Ampullae

Ossicles