Chordates
Chordates have a coelom and are a major branch of bilaterally symmetrical animals. We have looked at the major groups of the protostomes (molluscs, annelids, arthropods, etc…), now we will switch to looking at deuterostomes. Chordates are defined as having a notochord, pharyngeal slits, endostyle/ thyroid gland, dorsal hollow nerve cord, and post anal tail. The evolution of each of these structures has allowed for increased mobility, respiration, acquisition of nutrients, and reproduction. There are many organisms that display many but not all of these traits. One such group is the hemichordates. Hemichordates display half the characteristics of a chordates and half the characteristics of echinoderms (starfish, sea urchins, sea dollars, etc…). Chordates evolved early in the Cambrian period (about 530 million years ago), however the first real chordates most likely never left fossils because they did not evolve hard teeth and bones until later in the evolutionary timeline. Through half a billion years of evolution the basic body plan for vertebrates evolved leading to modern vertebrates.
Vertebrates
Vertebrates are true chordates possessing, at some point in their life, all five characteristics of chordates. Beyond these characteristics vertebrates also have a vertebral column and a true cranium.
Figure 1: Phylogenetic Tree of Vertebrates
Early vertebrates were all jawless fish, called agnathans. These fish were thought to be filter feeders using there large pharynx’s to suck in organic particles. Living decedents of these agnathans are lamprey and hagfish. Quickly (relatively from a geological stand point ~ 50 million years) after the rise of vertebrates the first jawed fish (gnathostome) evolved. This gave fish a more efficient way to capture food. With more efficient ways to capture food other characteristics began to develop. Fish became more mobile to capture food, they began fighting gravity and moving off the sea floor, and occupying different habitats. One early adaptation to aide in movement was the loss of calcification of cartilage into bone. This group of fishes still exists in sharks, skates, rays, and ratfish. However other adaptations were taking place leading to modern teleost (bony) fish with their swim bladders. By the late Devonian period (~350 million years ago) the seas were thriving with many types of fish competing with each other. This new pressure started a chain of events that drove evolution into the development of limbs and lungs that allowed movement onto land where predators were fewer and competition for resources was greatly reduced.
Activity:
Before dissecting any organism, get its overall weight in grams (table 1)
When dissecting your organism, be extra careful, you will also weigh shared organs across all organism to compare and answer questions ( Table 2).
Answer the questions as you compare the vertebrate anatomy.
Be able to identify the following structures from each provided critter:
Hag Fish
External
Barbel
Mucus openings
Mouth
Anus
Internal
Pharynx
Gill Pouch
Heart
Liver
Gall bladder
Intestine
Ovary
Shark
External
Rostrum
Ampullae of lorenzini
Gills
Spiracles
External nares
Male or female?
Dorsal Fin
Pectoral Fins
Heterocercal caudal fin
Pelvic fins
Cloaca
Internal
Bony Fish
External
Caudal fin
Dorsal fin
Anal fin
Anus
Pelvic fin
Pectoral fin
Gills
Opercula flap
Lateral line
Internal
Stomach
Liver
Air bladder
Intestine
Is it male or female?
Heart
Dissect heart-how many chambers?
Frog
External
Tympanum
External naris
Cloaca
Internal nares (inside mouth)
Vomerine teeth (inside mouth)
Maxillary teeth (inside mouth)
Internal
Stomach
Liver
Gallbladder (under liver)
Lung
Intestine (both)
Pancreas (behind/ under stomach)
Male or female? (and associated structures)
Kidney
Spleen
Heart
Dissect heart-how many chambers?
At the end of the Devonian period (~360 million years ago), vertebrates first started walking on land. The first tetrapod’s were transitional forms between aquatic and terrestrial likely spending most of their lives in shallow waters venturing on land to predate on insects or to lay eggs. These organisms lead to modern day amphibians. Once tetrapod’s ventured onto land many adaptations had to take place to accommodate the new and different environment on land.
Locomotion vastly changes from an aquatic habitat to a terrestrial one. This is displayed in the pattern of bones in the limbs. The general development of these bones in tetrapods is: one bone (ex. Humerus), two bones (ex. Radius/ulna), many bones (wrist), and hand. Breathing becomes very different in air rather than water. Development of lungs also changes the circulatory system greatly. In fish and amphibians reproduction takes place in water and eggs have a ready access to water. When organisms moved onto land other reproductive measures had to adapt. This creates two major splits in the vertebrate tree, the reptiles, that have hardened eggs to hold in moisture, and mammals, whom retain their offspring internally.
Be able to identify the following structures:
Mammal Anatomy
External:
Male or female?
Anus
Umbilical cord
Nipples
Urogenital opening
Internal
Hard palate
Soft palate
Incisor
Canine
Epiglottis
Tongue
Pharynx
Esophagus
Trachea
Thyroid
Heart (atrium, ventricles)
Lungs
Diaphragm
Liver
Gallbladder
Stomach
Spleen
Pancreas
Small intestine
Large intestine
Urinary bladder
Carotid artery
Kidney
Vena cava
Aorta
Iliac artery
Male or female? (and associated structures)
Table 1
Organism | Body Weight (g) | Heart (g) | Liver (g) | Small Intestine (g) | Large Intestine (g) |
---|---|---|---|---|---|
Hag fish | |||||
Shark | |||||
Perch | |||||
Frog | |||||
Pig |
Table 2
Organism | Heart/BW | Liver/BW | SI/BW | LI/BW |
---|---|---|---|---|
Hag fish | ||||
Shark | ||||
Perch | ||||
Frog | ||||
Pig |
THINK ABOUT WHAT YOU SAW:
How did the cardiovascular system change between Species? (number of Hearth Chambers and overall proportion of the body mass)
Why would one species have a larger proportional heart compared to another?
Which organism had the largest proportional liver? Why?
How did the digestive system change from aquatic to terrestrial? Why?
What other anatomical differences did you notice between species?