The Anatomy of Slugs
from Helen buttemer 1997
Student # _____
Ok you slugs (just kidding!)! Lets slide into our study of these slimy invertebrates!! You will note that a slug's body has four general regions.
Head Mantle Trunk Foot
Head - The head has some really cool things sticking out of it.
You can see that there are two pair of retractile tentacles on the head. The lower pair is shorter and used for smell (olfactory) and feeling (tactile). The upper pair is longer and used for seeing.
The mouth is on the lower part (ventral) of the head just aft of the lower tentacles. It has fleshy lips that are pushed apart when eating to expose its scraping tongue (radula).
The radula (scraping tongue) has many rows of hard teeth (chitinous like a crab shell!) on it. These teeth eventually wear away and are replaced over time. (Ever get licked by a cat? The slugs tongue is kinda like that!)
The pedal slime gland is just below the mouth. Its about 1 cm across on Banana Slugs. It releases mucus for slug movement.
On the right side of the slug's head, just below the tentacles, is the genital orifice. When visible it is a brownish depression or milk-white disk.
Mantle - Protection and Breathing (Respiration)
The Mantle looks sort of like a saddle on a horse. It is made of a thicker flesh then the head. When frightened or not active, the slug retracts its head into the mantle for protection. The Mantle is a fleshy fold of skin which forms the Respiratory Cavity (sort of like a lung). In snails the mantle also makes the shell... (In some slug species there is a little piece of shell in the mantle!)
On the right side of the mantle is an opening called the Pneumostome. The Pneumostome opens and closes to let air in and out of the "lung" (respiratory cavity). A muscular diaphragm forms the floor of the mantle cavity. This operates kind of like a bicycle pump, pulling and pushing air in and out of the pneumostome.
Under the mantle, just aft and across (lateral) to the pneumostome is the anal opening. This is where the slug gets rid of its wastes.
Foot - Just keep on sliding!!
The foot is very muscular. It forms the entire bottom (ventral) surface of the slug. A lengthwise band of muscle runs the entire length of the foot. Contractions of the foot from the tail to the head allow the slugs to travel forward on their food gathering expeditions. (Slugs can't move backwards though.) There is often a color pattern on the foot and the mucus may be colored too.
Trunk - Organ central - a visceral deal.
The trunk is jammed with organs coiled around one another. This arrangement is called a visceral mass. The visceral mass contains all the organs of the digestive and reproductive system. It has a tough transparent membrane enclosing them. Visceral mass is twisted because when it was developing it twisted everything to make it all lie within the whorls of its shell. But it is confused, it doesn't have one!
The slug's trunk ends in a mucus pore, which is usually clogged with mucus.
Inside the trunk under the portion covered by the left side of the mantle is its heart. The heart has just two chambers (we have four!)
Slugs have blood! Their blood contains white cells (ameobocytes) and hemocyanin, which carries Oxygen to the slug's cells and Carbon Dioxide away. (Some slugs have hemoglobin which is what humans use to do the same thing!)
The slug's single large kidney lies inboard from the heart. It filters the blood of wastes and tries to keep proper water balance in the slug. (Water balance = very difficult task!)
Reproduction - Slugs have it both ways
Slugs are hermaphrodities! This means slugs possess both male and female organs. They are only one sex at a time. Slugs start out being "males" because their sperm mature first. Once the sperm is expelled the female organs mature. In some species the slug mates, loses its penis, then becomes a female! The complications of mating in the Bannana slug are so great that, in some cases, the animals can only separate after sperm deposition by the mutual act of chewing off the partner's penis at the genital aperature!
How reproduction happens. A Hermaphrodite gland (ovitestis) produces both eggs and sperm. The vagina of the female receives the penis, and sperm of the male via the genital orifice. Sperm is stored in a pouch, called the spermatheca until the eggs are fertilized. Once fertilization occurs, an albumen gland puts the outside coating on the eggs and they are laid.
Nervous System - the slug has a simple nervous system.
Ganglia (nerve bundles - a way simple "brain") surround the esophagus. From there nerve cells spread out to other ganglia in the body. Nerve signals run up and down this simple pathway.
Digestion - (So that's where my flowers went!)
The slug's digestive system is pretty similar to humans. Its organs are set up in this order.
Lips, Mouth, Radula (teeth), Esophagus (food pipe), a huge Crop (this is a crushing organ cuz the teeth aren't all that effective in chopping food up), a Stomach with Digestive glands inject digestive fluids into the stomach to help further digest food, Intestines, Rectum, Anal opening.