Campbell, Biology 6th Ed - Chapters 32, 33 & 34, pgs 633-645, 672-717
Campbell, Biology 7th Ed - Chapters 32, 33 & 34, pgs 626-637, 665-709
Symmetry – general, structured body plan.
Coelom – fluid-filled body cavity surrounded by mesoderm-derived tissue.
Triploblastic – having three layers of cells during development: the endoderm, ectoderm, and mesoderm.
Deuterostomes – coelomate animals whose embryogenesis includes the blastopore developing into the anus of the animal.
Segmentation – division of the body into separate, sequential cavities.
Closed circulatory system – when blood is maintained in interconnected blood vessels and not mingled with other body fluids.
Cephalization – concentration of sensory and neural organs at the anterior end of an animal.
Metamorphosis – a restructuring of body shape and structure during development; typically from a juvenile to an adult form.
Filter-feeding – a method of heterotrophy, whereby a medium (typically water) passes by the animal and any suspended particulate items are “filtered” from the medium and ingested by the organism.
In the previous summaries on simple animals and protostomes, we explored much of Kingdom Animalia. In this summary we look at the remaining animals, those described as deuterostomes. Below is a review of the current classification of animals, based on morphology and development. You should familiarize yourself with the major anatomical and developmental differences that define animal phyla.
The traditional phylogeny is shown below. It is based on the following anatomical / developmental traits:
· Presence / absence of true tissues
· Type of body symmetry
· Presence / absence of body cavity (coelom)
· Pattern of coelom development (acoelom, pseudocoelom, true coelom)
The grades of animal evolution are as follows (italics – taxa described here):
1. No true tissues (only non-specialized cells) : Parazoa
2. True tissues (functionally-specialized cells) : Eumetazoa
A. Radial symmetry, diploblastic (2 germ layers during development) : Radiata
B. Bilateral symmetry, triploblastic (3 germ layers) : Bilateria
1) No body cavities or blood vascular system : Acoelomates
2) Body cavities and blood vascular system
a. Body cavity not enclosed in mesoderm : Pseudocoelomates
b. Body cavity enclosed in mesoderm : Coelomates
i) Early development includes spiral and determinate cleavage, mouth developing from blastopore, and schizocoely : Protostomes
ii) Early development includes radial and indeterminate cleavage, anus developing from blastopore, and eterocoely : Deuterostomes
Echinoderms (Sea stars)
Chordata (Tunicates, Lancelets, and Vertebrates)
This phylogeny of all animals is based on morphological differences and similarities.
Deuterostomes are radially or bilaterally symmetrical and triploblastic. Like protostomes, they are coelomates, possessing a true coelom.
As you learned in the previous summary, protostomes and deuterostomes differ in several characteristics of early development.
Phylum Echinodermata (Sea Stars, Sea Cucumbers, Sea Urchins)
Echinoderms represent the phylum most closely related to chordates. They are marine and mobile. They have secondarily evolved radial symmetry; that is, their ancestors were bilaterally symmetrical. They have only an oral and aboral surface. Echinoderms get their name (which means “spiky-skinned”) from the spines that project from their endoskeleton of calcareous plates. They have a thin epidermis over the endoskeleton. They have a tubular digestive system, but no circulatory or excretory system. Respiration occurs across the surface of the body. They have no segmentation. An interesting feature of echinoderms is the presence of a water vascular system that provides a means of locomotion.
You dissected sea stars in lab, noting the external morphology described above. The side of the sea star that you usually see is the aboral side.
Images of sea stars
Close up of the central body of the star fish
In addition to spines, sea stars have projecting pedicellariae that help keep the sea star clean.
On the oral side of the sea star are the mouth, ambulacral grooves and tube feet.
Mouth Ambulacral Groove
The digestive system of the seas star is tubular. When feeding, the seas star everts its esophagus and cardiac stomach to surround the material. The pyloric caecae are digestive glands located in pairs in each star arm that help digest food in the stomach. The food then enters the intestines and leaves the animal through the anus on the aboral side. Each arm also contains a pair of gonads.
Sea stars have a unique adaptation for locomotion – the water vascular system. Water flows around the animal through canals and into the tube feet. These tube feet allow for locomotion and adhesion to surfaces.
As in other radially symmetrical animals, the nervous system of the sea star lacks cephalization.
Some more echinoderms:
Sea urchin Sea cucumber
Phylum Chordata (Tunicates, Lancelets, Vertebrates)
All embryonic chordates share the following features: 1) dorsal, hollow nerve cord; 2) notochord; 3) pharyngeal gill slits; and 4) post anal tail. The phylum includes a few marine non-vertebrates and the vertebrates.
Subphylum Urochordata (Tunicates)
Tunicates, or sea squirts, are sessile, filter-feeding marine animals. The tunicate larvae are morphologically distinct from adults, being mobile and tadpole-shaped. Water moves through the larvae by entering the mouth and leaving through the excurrent siphon. This stage has all four chordate characteristics.
The larvae adhere to a surface and metamorphose into the adult tunicate. See the life cycle below.
The adult sea squirt is different in body shape from the larvae. It still has an incurrent and excurrent siphon; water is brought into the pharyngeal basket. Suspended material in the water is caught in mucus made from the endostyle and transported to the stomach. Gas exchange in tunicates occurs by diffusion across body surfaces, and the circulatory system is open. Gametes are released near the excurrent siphon and expelled into the water. Below is a photo of tunicates in their natural habitat.
Subphylum Cephalochordata (Lancelets)
Lancelets are mobile, fish-shaped marine chordates. Lancelets are also filter-feeders: they draw water in through the oral opening, into the pharyngeal basket, and out the atriopore. Behind the oral opening is a vestibule which traps food particles in mucus. Gas exchange occurs as water passes over the gill bars.
Lancelet; note the oral opening surrounded by tentacle-like oral cirri, and the gill bars.
The nerve cord of the lancelet ends at the anterior end in an enlargement, which represents a primitive brain. The circulatory system is closed. In the cross-section of Branchiostoma below, you can see the pharynx, gill bars, notochord and hollow nerve cord.
Vertebrates are some of the widest-known animals, including their most famous representatives – us! All four traits of chordates occur at some point of the development of vertebrates, although limited to early in development for some traits and groups. In addition, they have the derived trait of a vertebral column (backbone) surrounding the nerve cord. Vertebrates are highly cephalized and have an anterior brain encased in a bony skull. Segmentation and specialization are advanced. The endoskeleton usual includes two paired appendages. The circulatory system is closed.
There are seven classes of vertebrates, including three classes of fish, the amphibians, reptiles, birds and mammals.
Class Agnatha (Fish lacking jaws)
Lamprey Lamprey's mouth
Class Chondrichthyes (Cartilaginous fish)
Sharks, skates and rays
Class Osteichthyes (Body Fish)
Fish are aquatic vertebrates with appendages adapted for swimming. Gas exchange is accomplished by drawing water through the mouth and across the blood vessels of the gills.
In lab, you dissected a perch. The perch has an interesting trait – the lateral line. This line is located along the body of the fish and is sensitive to water pressure and electrical current. See the pictures below of the external anatomy of a perch.
As mentioned above, respiration in fish occurs using the gills. Large plates called opercula cover the gills. Refer to the diagram below which shows how water passes over the gills.
The digestive tract includes an esophagus, stomach, intestines and anus. The circulatory system is closed, with a two-chambered heart. Fish also have a swim bladder, which is an air-filled sac that helps maintain buoyancy.
Class Amphibia (Amphibians)
Amphibians are the most ancestral terrestrial vertebrates. They have many adaptations for life on land; however, they retain many ties to their aquatic ancestry. For example, many amphibians respire partially are exclusively through their skin, requiring that their skin be constantly moist (they also have lungs for respiration). In addition, 1) fertilization is often external, demanding that it occur in water, 2) amphibian eggs are non-amniotic (reptiles, birds, and mammals have amniotic eggs), making them sensitive to desiccation and requiring a moist oviposition site, and 3) many amphibians have a life stage that is aquatic (e.g. frog tadpoles) and may undergo metamorphosis.
In lab, you dissected a frog. The anatomies of the digestive and excretory systems are similar to the pig. In addition, the internal organs are in similar positions. The heart is three-chambered in amphibians, and lungs are small or absent. The urinary, intestinal, and reproductive openings collect in a common chamber, the cloaca, prior to exiting the body.
Virtual Frog Dissections
For more on amphibians and their diversity:
This class includes turtles (Testudines), snakes and lizards (Lepidosauria), crocodiles and their relatives (Crocodilia), and in some classification systems, birds (Aves), as well as a number of extinct groups. Reptiles (including birds!) are amniotes; that is, their eggs are protected from desiccation and other environmental problems by an extra membrane, the amnion, not found in the first terrestrial vertebrates (amphibians).
All reptiles have (or did have, in their evolutionary history) horny epidermal scales made of a particular kind of protein, paired limbs with 5 toes, skulls with a single occipital condyle, lungs instead of gills for respiration, and a 3 or 4 chambered heart. Their eggs are covered with a leathery or calcium-based shell (partially or completely lost in some species that give birth to live young), and fertilization occurs inside the female, rather than outside, as it does in most amphibians. Members of Reptilia generally share many additional traits, for example in their nervous and excretory systems, locomotion, and reproduction.
Birds are an extremely distinctive and successful clade, with an estimated 9000 species worldwide. Although descended from the dinosaurs, birds have evolved remarkable specializations for flight: a unique "one-way" breathing system, light yet strong hollow bones, a skeleton in which many bones are fused or lost, powerful flight muscles, and -- most importantly -- feathers. Like the reptiles, they are amniotes and sometimes are classified in Class Reptilia.
Birds are a monophyletic lineage, evolved once from a common ancestor, and all birds are related through that common origin. There are a few kinds of birds that don't fly, but their ancestors did, and these birds have secondarily lost the ability to fly.
Class Mammalia (Mammals)
Mammals are terrestrial vertebrates (tetrapods), and have evolved numerous features for life on land. Land adaptations in vertebrates are extensive, and include 1) limbs specialized for walking or flying, 2) water conservation mechanisms, 3) eggs protected against desiccation, 4) lungs for respiration. Mammals have additional derived traits that make them quite different from ancestral vertebrates, including 1) nourishment of retained embryos with a placenta (in placental mammals only), 2) mammary glands for nourishing dependent offspring, 3) endothermy, 4) fur for thermoregulation, and 5) functionally-specialized dentition (e.g. molars versus incisors).
In lab, you dissected a fetal pig. Some important traits that the pig has which the perch did not are: 1) legs with bones designed to aid in walking; 2) lungs for respiration, located under the rib cage; 3) an umbilical cord; and 4) a four-chambered heart. Why are these traits important for the pig, given what you know about its habitat and metabolism?
Virtual Pig Dissection
Other Mammal Dissections
Rat - Manitoba - Labs 15 & 16
-What about the echinoderms habitat likely facilitates respiration across the skin?
-Why do sea stars not need a cephalized nervous system?
-What four traits do all chordates share in common? When are these traits seen during the lifespan of the different chordates?
-Why does a lancelet have a small brain while the tunicate does not?
-What is the function of a fish’s operculum? swim bladder?
-How does the habitat of a fish, frog and mammal differ in terms of moisture availability? How is respiratory method related to this?
-Why do mammals have four-chambered hearts while fish and frogs do not?
-Compare and contrast the appendages of a fish, frog and pig. How are each type of appendage adapted for the particularly type of locomotion – that is, how does the form fit the function?
-What traits of amphibians are adapted for life on land? Which traits require use of water?
-Why have amphibians retained many ancestral traits associated with an aquatic habitat? Why have the “more advanced” terrestrial vertebrates not replaced amphibians on land?