Introduction and Learning Objectives
This final tutorial covering animals examines the deuterostomes. There are two major phyla in this group, the Echinodermata and the Chordata (Fig. 1). The Chordata contains three subphyla: Vertebrata, Cephalochordata, and Urochordata. We will explore the features that characterize these phyla and how these structures have evolved for many purposes. By the end of this tutorial you should have an understanding of the:
Figure 1. Animal Diversity and Body Plans (Click image to enlarge)
The Other Coelomates: Deuterostomes
To understand the evolutionary history of animals, scientists rely on several types of data including, but not limited to, the following: the overall morphology of the organism, DNA sequence comparisons, other similarities at the molecular level, and the developmental sequence of events in the embryo.
The sequence of events during embryonic development is the main characteristic used to distinguish protostomes and deuterostomes. Review Figure 2, which shows the major developmental characters that distinguish a protostome from a deuterostome. Recall that, in general, in protostomes the mouth forms first, whereas in deuterostomes it forms secondarily (the two names are based upon this difference). Another important distinction is how the coelom forms; in protostomes it forms from a splitting of the mesoderm, whereas in most deuterostomes the coelom forms from mesodermal outpocketings of the archenteron.
In this tutorial we will focus on the two major deuterostome phyla, Echinodermata and Chordata.
Figure 2. Protostome versus Deuterostome Development. (Click image to enlarge)
Representatives of the phylum Echinodermata are common inhabitants of coastal tide pools (you can see them in the saltwater aquarium at the HUB). Sea lilies, sea stars (Fig. 3), sea cucumbers, and most other echinoderms are sessile or slow-moving animals. Some representatives are instantly recognized by their five-fold symmetry (having rays and/or arms in fives or multiples of five). Even though the seemingly radial symmetry of these adult animals would lead one to question their placement in the bilateria, the bilateral symmetry of the larval stage of these animals indicates that they are members of the bilateria lineage. Furthermore, these animals are coelomates since they have a large, fluid-filled cavity lined with mesoderm and usually a complete gut. Many species of sea stars have the ability to turn their stomachs inside out by everting them through their mouths. This allows them to initiate the digestion of their prey before food enters their digestive tract.
A thin skin overlaying a hard, yet flexible, endoskeleton composed of calcium carbonate plates and spines characterizes the exterior of echinoderms (the name means spiny skin). These plates have a very complex arrangement. Electron microscopy of the surface of a sea urchin demonstrates this elegant arrangement, a sponge-like mesh which creates a plate that allows for special structures to protrude through the endoskeleton for locomotion, feeding, gas exchange, and protection. These protrusions, typically described as tube feet (sucker-like appendages), enable echinoderms to move and provide them with the ability to grip and manipulate objects or prey.
Figure 3. A Starfish. (Click image to enlarge)
Phylum Echinodermata: Classification
Figure 4. A Sea Star, a member of the Class Asteroidea. (Click image to enlarge)
Figure 5. A Sea Urchin, a member of the Class Echinoidea. (Click image to enlarge)
Figure 6. A Sea Cucumber, a member of the Class Holothuroidea. (Click image to enlarge)
Figure 7. A Brittle Star, a member of the Class Ophiuroidea. (Click image to enlarge)
What Do Humans Have In Common With Sea Urchins?
The relationships between humans and sea urchins may not be obvious, however, common embryonic features reveal their common ancestry. As you have learned, stages of embryonic development have an important place in the classification of animal phyla. Whether it is the fate of the blastopore, the number of germ layers (diploblastic versus triploblastic), or the formation and origin of the tissue lining the body cavity, embryonic origins provide information about the relationships among members of the Kingdom Animalia.
Chordates are a group of deuterostomes (Fig. 1) that show a number of complex adaptations. They have four major characters that distinguish them from other deuterostomes: a notochord, a dorsal nerve cord, pharyngeal slits, and a postanal tail (Fig.8).
The dorsal, hollow nerve cord is basically a sheet of ectoderm rolled into a tube. A chordate's nerve cord develops into the brain and spinal cord in adults.
Just below the nerve cord is the notochord, which exists in all chordate embryos, and some adults. The notochord is a long flexible rod that provides support. In vertebrates, it becomes part of the intervertebral discs in adults. Typically the muscles surrounding the notochord are arranged in distinct segments.
The pharyngeal slits are openings in the pharynx (part of the throat) of an animal. In the most primitive state these slits allow water to enter the mouth and exit without passing through the digestive system. In the more derived state the pharyngeal slits (and the tissue between them, the pharyngeal arches) serve various functions, ranging from gas exchange to food collection.
At some point in their development, all chordates have a tail that extends beyond the anus (a postanal tail). As with all of these basic chordate characteristics, the tail can be present in the developing embryo and absent in the adult (depending on the type of organism).
Figure 8. Chordate Body Features.(Click image to enlarge)
There are three subphyla within Phylum Chordata: Vertebrata, Cephalochordata, and Uroochordata. All cephalochordates and urochordates are invertebrates (animals without backbones), as were all of the animals discussed in the previous tutorials. All members of the Subphylum Vertebrata are vertebrates (animals with backbones).
Subphylum Cephalochordata includes the cephalochordates (lancelets), which are small marine animals that burrow, tail first, into the sand. In this position they are able to draw food particles into their mouths via their waving tentacles. Examine this illustration of the anatomy of an adult lancelet, Branchiostoma, (Figure 9), comparing it to the generalized chordate in Figure 8, and be prepared to answer a question about adult characteristics in the tutorial quiz.l.
Figure 9. Anatomy of an adult Lancelet. A marine cephalochordate. (Click image to enlarge)
Subphylum Urochordata includes the tunicates ("sea squirts"), which are marine animals that are typically sessile as adults. The lightbulb tunicates in Figure 10 are adhering to coral.
Figure 10. Lightbulb Tunicates. A marine cephalochordate. (Click image to enlarge)
The adult tunicate (Figure 11) does not appear to possess most of the standard chordate characteristics. However, the larval stage (Figure 12) clearly exhibits chordate characteristics; note the notochord, the hollow nerve cord, the pharyngeal gill slits, and the postanal tail. In adults the gill slits are modified for filter feeding. Molecular evidence supports the idea that the urochordates are the group most closely related to the vertebrates.
Figure 11. Anatomy of an Adult Tunicate. Note the lack of standard chordate characteristics in the adult tunicate. (Click image to enlarge)
Figure 12. Anatomy of a Larval Tunicate. Note the presence of standard chordate characteristics in the larval stage. (Click image to enlarge)
Phylum Chordata: Subphylum Vertebrata
Subphylum Vertebrata includes organisms with several basic features, in addition to the typical chordate characteristics. Vertebrates exhibit the most pronounced cephalization, in which the anterior portion of the nervous system (the brain) is enclosed inside a protective case (the skull). These animals all have a vertebral column (backbone), an axial endoskeletal system that forms dorsally to the notochord. This system serves to protect the dorsal nerve cord, support the head, anchor muscles, and protect internal organs. The anterior portion of this nerve cord is protected by a cranium in all of the vertebrates.
Figure 13 illustrates the evolutionary relationships among members of Phylum Chordata. As you look at the evolution of the group, note the acquisition of the more derived features: jaws, teeth, lungs, legs, amniotic eggs, and hair. We will explore these features (and their origins) in more detail in this tutorial.
Figure 13. Evolutionary Relationships Among Chordates. (Click image to enlarge)
Evolution of The Jaw
During vertebrate evolution, many modifications have occurred in the pharynx. If you have an opportunity to take a class in mammalian embryology, you will learn that many structures are derived from different regions of the embryonic pharynx (e.g., human nostrils originate from pharyngeal slits, which are present in the early human embryo). We will examine the evolution of the jaw so that you can appreciate how evolutionary biologists think these more derived structures evolved.
These jaw images (Figure 14) are arranged left to right, with the more ancestral state on the left and the more derived state on the right. In the ancestral state, endoskeletal elements are seen in the pharyngeal arches between the pharyngeal slits. It is likely that in this configuration the endoskeleton evolved to support the pharyngeal slits. However, in an event that probably involved a change in the growth of the more anterior arches, the endoskeletal elements became thicker and surrounded the mouth; primitive jaws appeared. These likely allowed the fish to capture food more effectively. In the most derived state (depicted on the right), further changes took place and these anterior bones became thicker and new bony protuberances appeared in the form of teeth. Teeth made predation more efficient, and are found in many vertebrates; most vertebrates that lack teeth are very specialized feeders, consuming food that does not need to be “handled”, including nectar, blood and termites/ants.
You should recognize this gradual adaptation of existing structures for new purposes as a reoccurring theme in evolution. The evolution of other pharyngeal structures, which gave rise to lungs and their derivatives, has also contributed significantly to vertebrate evolution.
Figure 14. Evolution of the jaw. (Click image to enlarge)
Evolution of Legs and the Amniotic Egg
Legs and amniotic eggs are two additional derived characters that have played a significant role in the evolution of vertebrates. The significance of legs in the terrestrial environment is obvious. How do evolutionary biologists think they evolved?
The endoskeletal system of vertebrates consists of the axial elements that are associated with the vertebral column. In addition, more lateral elements, termed the appendicular skeletal system, are found in some groups. The fossil record indicates that the appendicular skeletal system first appeared in a group of fish that have more robust fins with a main boney element, the "lobefin" fishes. (as opposed to the light, ray-like boney elements in the rayfin fishes) A progressive thickening of these elements occurred (Figure 15). The fossil record (and other data) indicates that the tetrapods, terrestrial vertebrates, arose from lobefin fish, and that legs evolved from these highly modified lobe fins.
Figure 15. The Evolution of Amphibians and Legs. Data suggest that amphibians arose from lobe-fin fish, and that legs arose from the highly modified lobe fins.(Click image to enlarge)
Early vertebrates arose in the oceans, where desiccation was not a problem. However, with the invasion of land about 365 million years ago, ancient vertebrates had another problem to face, water loss. This was similar to the problem faced by land plants. The vertebrates evolved a toughened egg that was resistant to drying, the amniotic egg (Fig. 16), which would have been selectively advantageous in a drier environment.
In reptiles, birds and mammals, the amniotic egg has several unique features, including a specialized membrane system that has many functions, including gas exchange and the transfer of vital nutrients to the developing embryo, and a protective and porous shell. The amniotic egg is name for the amnion, a membrane that surrounds the embryo and contains amniotic fluid, which bathes and protects the embryo.
Figure 16. The Amniotic Egg of Vertebrates. (Click image to enlarge)
This armadillo (Fig. 17) belongs to the vertebrate class Mammalia, as does this panther (Fig. 18). Mammals can be distinguished from the other vertebrates on the basis of several characteristics. All mammals have hair that functions (to some degree) for insulation, three bones in their inner ear that increasing hearing sensitivity, and mammary glands for milk production, and internal fertilization. Most mammals develop in the maternal uterus and are nourished by nutrients diffusing from mother to child across an organ called the placenta. Young marsupial mammals complete their development in a maternal pouch; marsupials include kangaroos, opossum, and Tasmanian Devils. A few animals (e.g., the echidna and the platypus) are monotremes, which are mammals that actually lay eggs and lack nipples.
Figure 17. An armadillo. A mammal. (Click image to enlarge)
Figure 18. A Panther. A mammal. (Click image to enlarge)
Humans are vertebrates, and we are subject to the same evolutionary principles that govern all life on the planet. However, we are a relatively new species. Although our genus Homo first arose almost 2.5 million years ago, modern humans, Homo sapiens (those that learned agriculture and demonstrated tool use and complex social structure) date back to a mere 150,000 to 200,000 years ago. While this may sound like a long time ago, consider that this represents only 5000 generations. This is not long on an evolutionary scale. This figure shows the major migration paths prior to 10,000 years ago, and all of us can trace our own ancestry back to one or more of these migrations (Fig. 19).
Figure 19. Major Human Migration Paths Prior to 10,000 Years Ago. (Click image to enlarge)
The deuterostomes include the echinoderms and the chordates. he echinoderms are an exclusively marine group that show five-fold radial symmetry as adult, but are bilaterally symmetrical as larvae. They have an endoskeleton and are covered with a spiny skin. They have a complex water vascular system that hydraulically powers their movement using tube feet.
The Phylum Chordata contains the subphyla Urochordata, Cephalochordata, and Vertebrata. Individuals within the phylum all have a notochord, a dorsal nerve cord, pharyngeal slits, and a postanal tail. The notochord may or may not persist in the adult, and the pharyngeal slits are modified in various ways in the different groups. The Urochordates are the probable ancestors of vertebrates.
After reading this tutorial, you should have a working knowledge of the following terms:
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