RAT DISSECTION LAB
CLASS: MAMMALIA “breast”
ORDER: RODENTIA “to gnaw”
There are about 4,440 species in the CLASS MAMMALIA. Mammals live on every continent and in every ocean. 95% of all mammals (including the rat) are placental mammals. The developing embryos grow inside the mother attached to a PLACENTA which provides oxygen and nutrients to the young, which are born alive.
Mammals, like birds, are ENDOTHERMS. They generate heat internally by breaking down food and burning it for energy. An ENDOTHERM can LIVE IN COLD CLIMATES and BE ACTIVE FOR EXTENDED PERIODS OF TIME. Because of its faster metabolism a mammal NEEDS MORE FOOD than the same size reptile.
One of the characteristics of mammals is fur or hair.
Heat is constantly escaping from a mammal’s body through its skin and exhaled air.
Because producing heat requires so much energy, mammals LIMIT THEIR LOSS
OF BODY HEAT to the environment with insulating layers of HAIR/FUR and layers
of FAT. Hair/fur is made from KERATIN (the same PROTEIN that makes reptile
scales, bird feathers, and your hair & fingernails). Fur also provides
CAMOUFLAGE for protection.
Unlike reptiles, the limbs of mammals DO NOT protrude at right angles to their
The TAIL is used for balance and support when sitting up to use the front paws.
Mammals have CLAWS, hooves, or fingernails.
Mammals show CEPHALIZATION (the concentration of their sense organs in the anterior end). Locate the forward facing eyes (for depth perception). Notice there is NO NICTITATING MEMBRANE over the eye as in amphibians, reptiles, and birds.
Another characteristic of mammals is SPECIALIZED
TEETH. All animals have teeth that correspond to
the type of food they eat. The name RODENTIA
comes from the Latin word meaning “to gnaw”.
Rodents such as the rat have 2 pairs of
CONTINUOUSLY GROWING INCISORS
(front teeth) which are kept at the
correct length by constant chewing.
VIBRISSAE (whiskers) are sensory organs which enable rats to feel their way through tight spaces in the dark. The external ear opening is surrounded by a PINNA (a flap of cartilage covered by skin) that can be rotated to direct sound into the ear. The TYMPANIC MEMBRANE is located inside the head and 3 bones (instead of one) now conduct sound waves from eardrum to inner ear. The EXTERNAL NARES (nostrils) are located near the tip of the snout and are used for respiration and odor detection.
Another one of the characteristics of mammals is that
they MAKE MILK to feed their young. Locate the NIPPLES
on the ventral side of your rat. These connect to MAMMARY GLANDS
(modified sweat glands) inside which produce milk. Males also have nipples, but
because they lack the necessary female hormones, their mammary glands do
not produce milk.
Locate the exit openings for the digestive, excretory, and reproductive body
systems. Like humans, male rats have 2 exit openings; females have 3 exit
Egg layers like reptiles and birds have a cloaca and a vent. PLACENTAL
MAMMALS (mammals that don’t lay eggs) have a RECTUM and an ANUS
instead. The digestive system has its own unshared exit opening (ANUS).
In addition, females have a separate Males have a SHARED UROGENITAL OPENING opening for reproduction and excretory that releases both SPERM and URINE. waste (URINE).
WHAT SEX IS IT?
The ideal temperature necessary for sperm production is actually a few degrees cooler than body temperature. So males have their TESTES suspended outside their bodies in a sac (SCROTUM) to keep them cooler. Testes outside the body in a scrotum are called TESTICLES.
Mammals are EUCOELOMATES. The space you see surrounding the internal organs is “true coelom” (body cavity lined on both sides by mesoderm). Mammals have a “new improvement”. . . the
DIAPHRAGM. This sheet of muscle separates the body cavity into two compartments: the THORACIC CAVITY (containing heart and lungs) and ABDOMINAL CAVITY (containing digestive, excretory, and reproductive organs). This muscle contracts to PULL AIR INTO the lungs (NEGATIVE PRESSURE BREATHING) and improve breathing efficiency. The MESENTERY membrane still surrounds and supports the digestive system. The yellowish structures are stored FAT which can be used for energy. DIGESTIVE In all of the animals seen thus far, digestion of food has begun in the stomach. MAMMALS START DIGESTING THEIR FOOD IN THE MOUTH. SPECIALIZED TEETH cut, grind, and shred the food and SALIVARY GLANDS produce digestive enzymes (AMYLASE) which breaks down starch into sugars and begin to chemically digest the food before it reaches the stomach.
Food moves from PHARYNX (back of throat) into the GULLET opening and down the ESOPHAGUS to the STOMACH. Stomach ACID secreted by the walls of the stomach and the stomach muscles work to break down food. The PYLORIC SPHINCTER muscle at the end of the stomach controls the passing of digested food into the SMALL INTESTINE.
Mammals now have 3 sections of small intestine. The upper portion of the SMALL INTESTINE closest to the stomach, (DUODENUM), middle portion (JEJUNUM), and final section (ILEUM). The SMALL INTESTINE receives bile from the LIVER and pancreatic enzymes (including trypsin) from the PANCREAS. DIGESTION IS COMPLETED HERE and NUTRIENTS ARE ABSORBED through the VILLI (small fingerlike extensions) lining the small intestine. A fan-like membrane called the MESENTERY holds the folds of the small intestine together.
At the junction of the small and large intestine is a large pouch (CECUM). Remember: vertebrates (including mammals) DO NOT produce the digestive enzymes needed to digest cellulose in plants. The CECUM pouch contains microorganisms that can break down plants. Humans have an APPENDIX, a vestigial (leftover non-functioning) cecum. The lower end of the small intestine leads into the LARGE INTESTINE (COLON), where undigested waste is collected and passed into the RECTUM before exiting through the ANUS. The excretory and reproductive systems in female mammals use different exit openings. Marsupial and monotreme mammals still have a cloaca.
The main functions of the lobed LIVER are to MAKE BILE, STORE GLYCOGEN and VITAMINS, and PROCESS NITROGEN WASTE and other TOXINS (poisons) for the kidneys to remove. BILE made by the liver is stored in a GALL BLADDER in most mammals. However, RATS DO NOT HAVE A GALL
! Bile is made as needed and released directly into the DUODENUM. The PANCREAS is an BLADDER
elongated organ located in the first loop of intestine between the beginning of the small intestine and stomach. It secretes PANCREATIC ENZYMES (including trypsin used in the small intestine to break down proteins), and INSULIN and GLUCAGON (to regulate blood glucose levels).
The GLOTTIS, in the back of the throat is the opening to the respiratory system. The EPIGLOTTIS (a movable flap of tissue) covers the opening to the respiratory system and keeps food from “going down the wrong tube” when swallowing. The TRACHEA (with cartilage rings in keep the airway open) splits into 2 BRONCHI. The respiratory organ in mammals is the lungs, which have many small individual air sacs called ALVEOLI (sing.ALVEOLUS) to increase surface area for greater gas exchange. The DIAPHRAGM (a sheet of muscle below the ribcage contracts to pull air into the lungs. A voice box is located in the trachea and
makes sound as air passes through.
The endocrine system in mammals is the most complex. The THYROID GLAND regulates metabolism, but many other ENDOCRINE GLANDS such as the PITUITARY, ADRENAL, and PARATHYROID GLANDS also help to control other body organs, growth, development, and function.
Mammals excrete nitrogen waste in the form of UREA, which is toxic and must be diluted with water. The KIDNEYS, which lie on either side of the spine against the dorsal body wall, are the primary excretory organs. The kidneys filter NITROGEN WASTES (UREA) from the blood, and dilute it with water to make URINE. Urine flows from the kidneys through URETERS to the URINARY BLADDER for storage. Urine travels in the URETHRA from the bladder to the exit. In addition to removing nitrogen waste, the kidneys also REGULATE THE ION/WATER BALANCE in the blood and tissues (OSMOREGULATION).
Like birds, mammals have a CLOSED CIRCULATORY SYSTEM with 2 LOOPS and a FOUR CHAMBER HEART surrounded by a PERICARDIAL MEMBRANE. Locate the RIGHT ATRIUM, LEFT ATRIUM, LEFT VENTRICLE, and RIGHT VENTRICLE. The SINUS VENOSUS or CONUS ARTERIOSUS have disappeared.
PULMONARY CIRCULATION: The
PULMONARY ARTERIES carry LOW OXYGEN blood to the
from the heart to the lungs and the PULMONARY VEINS
return blood return HIGH OXYGEN blood to the heart.
The SYSTEMIC CIRCULATION carries oxygenated blood from the
heart to the muscles and body organs and brings deoxygenated
blood back to the heart, just like amphibians & reptiles.
(RENAL-to kidneys; CORONARY-to heart; HEPATIC-to liver).
Remember adding a second loop has the advantages of faster
blood flow to the body organs and more efficient delivery of
ARTERIES carry blood away from heart; VEINS return blood to heart; CAPILLARIES carry out gas/nutrient/waste exchange. The SINUS VENOSUS and CONUS ARTERIOSUS have disappeared. Largest vein returning to heart from body = VENA CAVA; Largest artery leaving heart to body organs = AORTA.
Low oxygen blood returning from the body
enters the RIGHT ATRIUM. At the same time
high oxygen blood returning from the lungs
enters the LEFT ATRIUM. When the atria
contract, blood is sent to the corresponding
VENTRICLE. The right side of the heart only
carries low oxygen blood. The left side only
carries high oxygen blood.
The LEFT VENTRICLE is thicker and stronger
than the RIGHT ventricle because the left side has to pump blood a farther distance out to all the body parts. The right ventricle just has to send blood to the lungs, which are close by.
The SPLEEN still makes, stores, and processes blood cells. Unlike other vertebrates with red blood cells the RED BLOOD CELLS in mammals have lost THEIR NUCLEI to allow more room for oxygen transport.
Rats are PLACENTAL MAMMALS. Males have TESTES that make the sperm. The sperm mature and grow their tails as they pass through the EPIDIDYMIS (coiled tubules on top of the testes). The SEMINAL VESICLES add fluids to sperm. Mature sperm passes through tubules called VAS DEFERENS to the URETHRA and leaves the body through the PENIS. Males have a PENIS to deposit the sperm inside the female’s body, which increases the chances of fertilization.
A female rat has paired OVARIES that produce the eggs, which are carried to the UTERUS in FALLOPIAN TUBES (OVIDUCTS). Fertilization is INTERNAL and occurs in the Fallopian tubes. If fertilization occurs, the developing embryo grows inside the mother’s UTERUS, nourished by a
PLACENTA. Oxygen and nutrients diffuse from mother to baby AND nitrogen waste and carbon dioxide diffuse from baby to mother across the close placenta connection. Young are born alive and pass out of the body through the VAGINA.
The reproductive pattern seen in rats (and most other mammals) is VIVIPARITY and most offspring are born looking like smaller versions of the adult (DIRECT DEVELOPMENT). After birth MAMMARY GLANDS in the female produce milk to feed their young.
CLASS ___________________________ _____________________________
ORDER __________________________ _____________________________
Which bile storage organ (found in other mammals,
birds, reptiles, and amphibians) is missing in rats? ________________________
How are the incisors of a rat different from yours?
Why would this be an advantage to an animal that can’t see a dentist?
Which structure is located at the junction of the small and large intestines? ____________________
What is the function of the CECUM? ____________________________________
Explain why the LEFT VENTRICLE is larger and the muscle stronger than the RIGHT VENTRICLE.
How are the exit openings for digestive, reproductive, and excretory different in male and female
MALE have _____________________________________________________
FEMALES have __________________________________________________
List the sections of small intestine in order starting at the stomach.
_____________________ ______________________ ____________________
Tell 3 ways REPRODUCTION in mammals is different than in any other animals you have seen so far.
Tell 3 ways that digestion of food in mammals is “new or improved” compared to amphibians or reptiles.
What new adaptation do mammals have to help them breathe more efficiently?
What is different about the RED BLOOD CELLS
of mammals compared to other vertebrates? ____________________________
In mammals nitrogen waste is excreted by the kidneys in the form of:
AMMONIA URIC ACID UREA
In all of the animals you have seen so far the testes (when present) have been located inside the body
cavity. Explain why mammalian testes are located outside of the body.
MAMMALS have a ____ chamber heart and a ______ loop circulatory system. (how many?) (how many?)
MAMMALS have a _____________________ heart and a __________________ spinal cord.
DORSAL VENTRAL DORSAL VENTRAL
How are they different?
How are they alike?
What is the difference? FUNCTION
SUBSTANCE MADE BY ? FUNCTION
Causes cells to release stored glycogen into
bloodstream as glucose
Causes cells to take up glucose from blood
and store it as glycogen
Breaks down fats
Breaks down proteins
Breaks down starch
All cells, especially LIVER Storage form of glucose used by animal
cells to save energy
THORACIC CAVITY ABDOMINAL CAVITY
Body systems found there?
Which part is it?
Gland in the mouth that starts digestion with amylase to break down starch _____________________
Tube that carries food to stomach ________________________
Digestive organ where acid & muscles work to break down the food _________________________
Large pouch at the junction of the small and
large intestine which contains microorganisms
that help to digest plant material ___________________________
Part of small intestine where bile and trypsin are added and digestion is finished _________________
Parts of small intestine where nutrients are absorbed _________________ & ___________________
Absorbs water from and concentrates digestive waste ___________________________
Holds digestive waste waiting to exit the body ___________________________
Holds excretory waste waiting to exit the body ___________________________
Dividing wall that separates the ventricle into two sides ______________________
Tubes that carry sperm from epididymis to urethra ________________________
Tubes that carry urine from kidneys to urinary bladder _______________________
Tube that carries urine from bladder to exit _____________________________
Coiled tubes where sperm mature and grow tails ___________________________
Muscular reproductive organ where the embryo attaches
and grows receiving nourishment through the placenta _______________________
Regulates heart rate and metabolism _____________________________
Main blood vessels that carry blood to lungs and return to heart _____________________________
Main blood vessel that carries blood leaving heart to body organs _____________________________
Main blood vessel that carries blood returning to the heart from the body ______________________
Fingerlike extensions inside small intestine
that increase surface area to absorb more nutrients _______________________