Squid (Teuthida)

Classification: Cephalopoda Class of Mollusca Phylum


There are more than 300 species of squids, and it is believed that many more species of squid are out there that have been undiscovered. They live in water of all different temperatures and can be found in fresh or salt water. They can range from 24 inches long to 40 feet in length. It is believed that the squid is one of the invertebrates that has the highest level of intelligence.

Dissection Procedure

The first step in dissecting a squid is to examine the external anatomy. You will see the dorsal which is the back side, the eyes, the mantle, the head region that contains the tentacles. The siphon, beak and mouth are located on the ventral side of the squid. The next step is the internal anatomy of the squid. Making an incision in the mantle you will probably see one of the most obvious structures first, which is the ink sac in the middle of the body cavity. You will see the gills, the gill-heart, and the nidamental gland. Lifting the nidamental gland you will see the stomach, which is small enough to miss. You will see the water jet that has the retractor muscles that are used for the squid's movement. Pushing the organs of the coelom aside, you will see the pen structure of the squid. The pen is a hard structure that stabilizes the squid when its swimming.
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Nervous System

Squids exhibit relatively high intelligence among invertebrates. A squid's brain consist of three parts; two optic lobes and a central ganglion. The central ganglion is a collection of very soft nerve tissue that surrounds the esophagus. So every bite the squid takes goes through the brain! The brain is protected by a case that looks like it is made of cartilage but it is a smiler protein.
Comparing the nervous system of the squid and the nervous system of a shark, they have no similarities and many differences.

External Anatomy

The main body mass is enclosed in the mantle, which has a swimming fin on each side. The skin is covered in chromatophores, which allows the squid to change color to suit its surroundings. Under the body are the openings to the mantle cavity, which contains the gills. At the front of the mantle cavity lies the siphon which the squid uses for locomotion. You can't forget the tentacles which are used to collect prey.
Squid Dissection: External Anatomy
Comparing the external anatomy of a squid and the external anatomy of a shark they do have a few things in common. Squids and sharks both have fins and gills since they are both marine animals.

Respiratory System

Water and oxygen enter the squids body through the mantle. The respiratory exchange between the carbon dioxide and the oxygen occur in the gills. The oxygen is then carried throughout the tissues by the blood and the carbon dioxide and water are expelled from the body through the siphon.
Comparing the respiratory system of a squid and the respiratory system of a shark there are many similarities. Since squids and sharks are both aquatic animals, they both rely on gills for their oxygen intake.

Digestive System

Squids have a complex digestive system. The stomach in a squid is found roughly midpoint of the visceral mass. From there the food moves into the caecum for digestion. Food then goes to the liver found at the siphon end for absorption. Solid waste is then released from the rectum.
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Comparing the digestive system of a squid to the digestive system of a shark, they do not have any similarities at all.

Skeletal System

Squids have no skeletal system. The only cartilage of the squid is the pen. The pen is a feather shaped internal structure that supports the squid's mantle and is a site for muscle attachment.
Comparing the skeletal system of a squid to the skeletal system of a shark they are very much alike. Both the squid and the shark do not have a skeletal system and both animals possess cartilage.