8.27.4 Lesson: Ectotherms

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H.1 – Identify the characteristics of Kingdom Animalia and its various phyla.



Air sacs of the lungs involved in gas exchange


Prolonged dormancy of an animal during hot, dry periods


Prolonged dormancy of an animal during the winter

Jacobson’s Organ

Chemical receptors located in the roof of the mouth of some reptiles, such as snakes

Lateral Line

Line of sensory organs along the side of a fish used to detect pressure and vibration

Swim Bladder

Gas-filled sac found in the bodies of bony fish used to control depth



Temperature Regulation

Not all organisms are able to regulate their own body temperatures. Instead, they rely on external sources for heat. Often considered “cold-blooded,” these animals depend on the environment to keep their temperatures within stable ranges. For instance, reptiles such as lizards, snakes, and geckos lie in the sun to warm their bodies and in the shade to cool themselves. Today, we will take a look at several different classes and orders of ectotherms.

Class Osteichthyes

Class Osteichthyes is a group known as the “bony fish.” Characteristics of this class include:

  • Bone structure throughout body with many vertebrae
  • Scales can be present or absent
  • Fins are usually paired; types include pectoral, pelvic, anterior and posterior dorsal, anal, and caudal
  • Teeth can be present or absent
  • Presence of jaws
  • Respiration occurs through gills that are covered by an operculum, which serves as protection

A classic representative of a bony fish is the white perch, seen in the photo below. A fish swims by using the movement of both its body and tail, and regulates its depth through the use of a swim bladder. A fish can increase or decrease the air in the bladder based on the desired depth.



Some fish have scales that cover their entire bodies and grow as the fish does. Scales have rings that can be used to determine the age of the fish, similar to how rings in wood can determine the age of a tree. However, not all fish have classic scales. Instead, some have bony plates called scutes, like the sturgeon in the image below (left), or the catfish in the image to the right, which does not appear to have plates or scales at all.



Below is a video of a White Perch dissection, which takes you step-by-step in identifying the structures of the fish.




Fish have a variable diet, feeding on organisms such as coral, plants, and other fish. Food enters through the mouth, travels down the throat and into the stomach. Pyloric caeca, which are tubular pouches located between the stomach and intestine, hold digestive juices that aid in breaking down the food. The liver is also near the stomach and secretes bile, which is necessary for the digestion of fats. Extra bile is stored in the gallbladder. Food continues to travel through the digestive system, passing through the pancreas, which secretes digestive enzymes as well. Waste is released through the anus.


Fish have a series of gills that have a protective covering called an operculum. The gills, which are thin plates with a blood supply, allow for the exchange of gases such as oxygen and carbon dioxide. The fish draws water into its mouth and forces water over the gills, allowing for the uptake of oxygen and the release of carbon dioxide back into the water.


Fish have a closed circulation system with a two-chambered heart consisting of one atrium and one ventricle.

Nervous System

Like all vertebrates, fish have a brain and spinal cord. The brain is divided into three sections that each serve a special function:

  1. Forebrain: Responsible for the ability to smell
  2. Midbrain: Processes vision, learning, and motor responses
  3. Hindbrain: Coordinates balance and movement

Salmon are an example of fish that have an excellent sense of smell. In fact, using the strength of this sense, these fish migrate back to the location of their birth during seasons of spawning.

In regards to the sense of hearing, fish use a special organ on the side of their bodies called the lateral line to sense vibrations in the environment.


Fish can reproduce either internally or externally. In external reproduction, the female lays the eggs and the male covers them with a coating, called milt, which contains sperm.

Class Agnatha

Class Agnatha is a group known as the “jawless fish,” and includes organisms such as the sea lamprey seen below. These fish are considerably different from the previous class of bony fish:

  • Scales are absent
  • Fins are unpaired
  • Skeleton is composed of cartilage
  • Sucker mouth
  • Only external reproduction methods are used


Watch the clip below on jawless fish.



Class Chondrichthyes

The next class, class Chondrichthyes, is a group known as the “cartilaginous fish.” Examples include sharks, skates and rays. These organisms, like those described above, also have cartilage in place of bones. However, they are different from the lamprey in that they have jaws and paired fins.


The shape of a shark’s body allows for fast, efficient swimming. Although the body of a shark appears smooth, it is actually made up of scales that interlock and push water down the shark, reducing drag and friction. In addition, the shark has the same fins as bony fish, however, the dorsal part of the tail fin is much larger.

Other characteristics of sharks:

  • They have thousands of teeth that replace themselves if lost.
  • They pump water over their gills in the same manner as other fish.
  • They have high sensitivity to their environment, especially smells and slight vibrations. In fact, they can sense electrical activity from other animals.

Skates and Rays

Skates and rays have a unique, flattened shape. They skim across the ocean floor as they feed, using their mouth which is located on the ventral or underside. Some species of these organisms have a long tail that contains spines, which are dangerous to other organisms. The largest ray, the manta, can grow up to 20 feet across!

Class Amphibia

Members of class Amphibia have a dual life: half is spent in water and the other half is spent on land. In most cases, it is the immature form that lives in the water and the adult that lives on land.


The life cycle of an amphibian begins when the terrestrial female lays eggs in water. The appearance and number of eggs differs according to the species of amphibian; there could be a single egg or thousands of eggs. The male then covers them with milt and the fertilized eggs develop into the larval stage in the water. Depending on the organism, it either develops into a tadpole or a mudpuppy. The larval form has gills and continues to grow and develop into the adult form with lungs, eventually leaving the aquatic environment.

Watch the brief clip below that depicts the development of a tadpole into a frog.



Special Adaptations

Some members of this class have special adaptations that allow for self-defense. One such adaptation comes through the use of skin secretions. These secretions may be poisonous or simply taste bad to prevent predator attacks. False coloration is another adaptation. Poisonous species are typically brightly colored; however, non-poisonous species can be found as brightly colored as well in order to avoid attack from predators.


Gas exchange can occur in multiple ways during the life of an amphibian. The first method involves the use of gills in the larval form. The second method occurs in the adult stage through the use of lungs. Additionally, the moist skin and the mouth/throat of amphibians are rich with blood vessels that are close to the surface and allow for the exchange of oxygen and carbon dioxide.


The amphibian circulation system involves a three-chambered heart that consists of two atria and one ventricle.

Amphibians are ectothermic. In cases of cold temperatures, they practice hibernation, slowing their body systems down almost to the point of stopping. In cases of hot temperatures, the animal undergoes estivation, which is also an inactive state that resembles a deep sleep.


  • Order Apoda: Caecilians
    • Limbless, serpentine amphibians

  • Order Caudata: Salamanders
    • Lizard-like amphibians with short limbs and the presence of a tail


  • Order Anura: Frogs and toads
    • Short-bodied amphibians without presence of a tail

Class Reptilia

Another class of ectotherms is composed of reptiles. Characteristics of these animals include:

  • Use of lungs to breathe oxygen starting at birth
  • Residence in aquatic or terrestrial environments
  • Heart with three or four chambers
  • Laying of shelled eggs
  • Specialized protective skin covering the body


Reptiles have lungs from birth and therefore do not require gills in early development. Even aquatic reptiles, such as sea turtles, have lungs with small sacs called alveoli that serve to exchange gases.


Most reptiles have a three-chambered heart: two atria and one ventricle. Crocodiles and alligators have four-chambered hearts.

Nervous System

The nervous systems of reptiles contain the same basic components as that of amphibians, however, the reptile cerebrum and cerebellum are slightly larger. Sense organs are well developed in reptiles. For example, most reptiles have an ear drum or tympanum that allows for the conversion of sound waves into audible noise. One obvious exception is the snake, which lacks external ears and instead has Jacobson’s organs. These are chemical receptors in the roof of the mouth. The tongue of the snake senses chemicals from its surroundings and sends the signals to the Jacobson’s organ. This method is helpful in feeding, reproduction and evading predators.


  • Order Squamata: Snakes and Lizards
  • Order Testudinata: Turtles
  • Order Crocodilia: Crocodiles and alligators
  • Order Rhynchocephalia: Tuataras (see image below)


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