Lab 6 - Echinoderms and
Introduction to Echinoderms
Our closest cousin among the invertebrates is a most unlikely taxon, the echinoderms (Phylum Echinodermata, = spiny skin; 6,000 sp ) Echinoderms are eucoelomate deuterostomes. They show a superficial five part (pentamerous) radial symmetry. The larvae are bilaterally symmetric, cephalized, and motile, but they develop into sessile or sedentary radially symmetric adults.
All echinoderms are marine. They have a calcareous endoskeleton, consisting of numerous small plates covered by a thin epidermis. They are probably the first animals to have evolved an endoskeleton derived from mesodermal tissue. Numerous small spines project from the surface of the body. Echinoderms have an open circulatory system, and respiration and excretion occur by means of dermal gills, small finger-like projections of the skin that stick out near the base of the spines on the surface. The large coelom also functions in circulation and in respiration. Mixed in with the spines and dermal gills on the surface of the animal are numerous small pincers on tiny stalks, structures called pedicillaria. These can snap shut on tiny prey, and help keep the animal's skin clear of any small settlers (they repel boarders).
Echinoderms move by odd little hydraulic structures called tube feet. Each tube foot has a small bulb called an ampulla. The ampulla squeezes water into the tube foot to stretch it out, with a one way valve keeping it from returning to the radial canals until the ampulla relaxes. Longtitudinal muscles in the feet contract to shorten them, pulling the animal along. Water enters the animal through a madreporite, a tiny sieve plate that keeps out pieces of debris. Water passes into a ring canal, out into a series of radial canals, and finally into the tube feet. Tube feet can function in both locomotion and in feeding.
Echinoderms have no brain, or central nervous system, consistent with their return to a sedentary life with a radially symmetric body plan. The nervous system consists of a simple nerve ring, with five branches to innervate the arms. Their senses are rudimentary, including light sensitive eyespots and sensory tentacles (modified tube feet) at the tips of the arms, and small patches of cells sensitive to chemicals or touch.
They have an unusual type of connective tissue, mutable or catch connective tissue, which can change consistency at will, from very hard to very soft. This is what allows starfish to flex their arms, or drop an arm if attacked by predators. Catch connective tissue also solidifies to lock the spines of urchins into their defensive position. Asexual reproduction occurs by splitting or fragmentation. Sexes are separate, with external fertilization. They have great regenerative powers; one arm can regenerate an entire starfish!
There are five living classes, but over 20 extinct classes of echinoderms. The ancestral echinoderm was probably an animal like the sea lily, which resembles an upside-down starfish on a stalk. The tube feet and water-vascular system originally functioned in filter feeding. Some echinoderms returned to an "active" existence, detached and flipped over (mouth side now down), with the tube feet now functioning in locomotion.
Class Asteroidea - starfish
Class Echinoidea - sea urchins, sand dollars
Class Ophiuroidea - brittle stars
Class Holothuridea - sea cucumber
Class Crinoidea - sea lilies
Observe the echinoderms on display. Try to visualize the basic starfish body plan as you look at the different classes. Think about how the basic starfish body has been modified in each class. What are the different feeding methods of the various classes? How do their physical adaptations reflect their method of feeding and diet?
The starfish has one big advantage over any other animal for dissection. If you screw up one of the arms, you get four more chances! Look for the following external features: arms, spines, madreporite, tube feet, ambulacral groove, ambulacral spines, mouth. You might spot the teeny tiny anus at the top center of the disc. Or you might not....
Cut across one of the arms, about halfway to the tip. Observe the extensive gonadal tissue lying in the coelomic cavity. The gonads connect to the sides of the arm near the point where the arm joins the disc. Even though sexes are separate, it is nearly impossible to tell male from female without microscopic examination. The feathery gonads lie under the pair of digestive glands that fill much of the arm. Put your starfish dorsal side up, and choose an arm to cut. Using your scissors, carefully cut along the mid dorsal line of the arm until you reach the central disc. Gently scrape the overlying tissue of the arm to expose the upper side of ambulacral groove and the tube feet. Note the ampullae on top of the tube feet, and the prominent radial canal.
Cut around the very outer edge of the disc, taking care to cut around the area of the madreporite. Gently remove the epidermis of the disc, without disturbing the madreporite. Look for the stone canal and ring canal (very hard to spot). The tiny intestine at the center of the disc (anus) will hold the tissue back, so you may need to snip through it. The intestine connects to the flat pentagonal pyloric stomach, which sits on top of the larger cardiac stomach, which fills most of the center of the disc. Try to find the pyloric duct that extends from the pyloric stomach into each of the arms.
Class Asteroidea - (1,500 sp.), starfish
Starfish are important marine predators. They are wolves in slow motion. Most have five arms. Note that the radial symmetry is only superficial, due to the presence of the madreporite. Some starfish can actually feed on bivalves by extruding their cardiac stomach. They can squeeze through an opening a mere 1/10 mm wide, within the natural tolerance of the irregular edges of bivalve shells.
Echinoids lack arms, but still show the characteristic five-part symmetry of the other echinoderms. You can clearly see the five rows of tube feet on the shells you will see in lab. There are over 5,000 fossil species of sea urchins. They are well protected by sharp spines, attached to the endoskeletal plates. These spines can get to be very long. Sea urchin spines are movable, and help the urchins crawl about. Sand dollars are sedentary echinoids. Echinoids feed by scraping algae off the substrate with their sharp teeth.
Members of this class resemble starfish, but their long arms are extremely brittle. Tube feet are modified for filter feeding on microscopic plankton.
Note the five part symmetry shown by the rows of tube feet. Endodermal plates are greatly reduced to a few small and scattered pieces inside the leathery epidermis. The mouth is surrounded by tentacles, which are actually modified tube feet. Sea cucumbers feed by snaring plankton in the mucus coating on their tentacles. They bring the tentacles into the esophagus to wipe them clean, recoat them with mucus, and feed some more. YUM! Sea cucumbers are considered a great delicacy in the orient (trepang or bêche-de-mere). They have a unique defensive mechanism. When threatened, they can evert sticky stinky hairs from their anus. Enough said...
Sea lilies are an ancient group, going back about 530 mya. They were thought to be extinct until they were rediscovered growing on the ocean floor. In sea lilies, the mouth and anus are both on the upper surface on a small disc, with the arms located along the edge of the disc. Crinoid tube feet are modified for filter feeding.
Some starfish (Crown of Thorns) cause extensive damage to coral reefs.
Sea cucumbers are an Oriental delicacy, supporting a multi-million dollar seafood industry.
What does the shape of the larvae suggest about the early evolution of echinoderms?
How are each of the classes of echinoderms derived from the basic starfish body plan?
How does the radial symmetry of echinoderms relate to their life style? Aren't all higher animals bilaterally symmetric?
We now turn to the last phylum of animals, one that dominates the deuterostomes as thoroughly as arthropods dominate the protostomes, the Phylum Chordata (42,500 sp.). Chordates are eucoelomate deuterostomes, and probably share a common ancestor with echinoderms. Three important characteristics unite the Phylum Chordata. At some point in their life cycle, all chordates have a notochord, a dorsal hollow nerve cord, and pharyngeal gill slits. A notochord is a flexible supporting rod of cartilage, although in most adult chordates the notochord is replaced by a vertebral column. The dorsal hollow nerve cord ultimately forms the spinal cord and the brain.
The pharyngeal gill slits appear in all chordate embryos, an echo of our distant origin in the sea, but are usually lost in the early development of the organism. Primitive chordates evolved small slits opening into the pharynx. By contracting the pharynx, the animal could draw water into its body and over the gill slits. These slits originally functioned in aiding respiration and capturing food by filter feeding. Smaller, more primitive vertebrates could rely on diffusion for gas exchange, but larger and more active forms required more surface area to allow rapid exchange of gases. Chordates evolved gills, sheets of highly folded tissue in the spaces between the gill slits, tissues with a very rich blood supply to exchange gases. Gill arches were reinforced with cartilage to help hold them open. Over time, the area between the gills, or the gill arches, became ossified (turned harder) and migrated slightly forward to form the first primitive vertebrate jaw. Vertebrates could now bite and chew their prey, and were no longer limited to filter feeding as a way of life.
Phylum Chaetognatha - arrow worms
Phylum Hemichordata - acorn worms
Subphylum Urochordata - tunicates
Subphylum Cephalochordata - lancelets (Amphioxus)
Subphylum Vertebrata - vertebrates
Class Agnatha - lampreys
Class Chondrichthyes - sharks, rays
Class Osteichthyes - bony fishes
Class Amphibia - frogs, toads, salamanders
Class Reptilia - snakes, turtles, crocodilians
Class Aves - birds (dinosaurs)
Class Mammalia - placental (humans), marsupial (kangaroo), monotremes (egg layers - platypus)
Phylum Chaetognatha - (100 sp., fr Gr. khaite = hair, gnathos = jaws), arrow worms, Sagitta
I doubt if anyone here has ever noticed one of these worms while swimming in the ocean. These tiny little predators are only 6-7 cm. long, and are completely transparent. But they are incredibly abundant. They are the most abundant carnivore in the ocean. They are the tiger sharks of the plankton. The tiny moveable hooks that surround the mouth, and give these creatures their name, are used to capture prey. Prey are injected with a tiny jolt of tetrodotoxin, the same paralytic poison found in some Japanese puffer fish. They lack circulatory, respiratory and excretory organs, relying entirely on diffusion. They are living fossils, going back essentially unchanged for about 500 my. They represent a very early branch on the chordate tree.
These marine worms are another ancient group, evolving about 450 mya. They may be the first deuterostomes on Earth. They range in size from 2 cm to 1.5 meters. They share some of the fundamental characteristics of the chordates, which we'll review later, such as a dorsal hollow nerve cord and gill slits. We used to think they also had a notochord, another chordate trademark, but closer study revealed this hypothesis to be wishful thinking. They live in U-shaped burrows in the ocean floor. Notice the slits in the side of the pharynx. These pharyngeal gill slits are used for gas exchange and feeding. This obscure little structure will eventually give rise to the vertebrate jaw, a marvelous example of evolutionary constraint - evolution is constrained to run in certain channels. All subsequent evolution has to start with what's already there. They share a common ancestor with echinoderms, a fact we deduce from their similar larval forms (dipleurula larvae) and other developmental similarities. This larval form, incidentally looks strikingly similar to the trochophore larvae of annelids and molluscs.
Subphylum Urochordata - (1,250 sp.), tunicates
Tunicates are sessile, marine organisms. They are covered with a cellulose cloak, or tunic, which gives this group its name. They exchange gases and filter feed by means of their pharyngeal gill slits. They rely on two prominent siphons, an incurrent and excurrent siphon, to pull water through their bodies. The pharynx is lined with cilia, which draw water in. The suspended organic particles stick to a layer of mucus in the pharynx, and are later eaten. These siphons are convergent with mollusc siphons. Tunicates look a bit like molluscs, and a bit like a transparent sponge, and may even function like these organisms, but these similarities are entirely superficial, and the three groups are not directly related. Although these curious animals don't especially look like us, they are very derived from their presumably bilateral and motile ancestors. The larvae of tunicates looks very much like a little tadpole. One of the strongest theories of vertebrate origins suggests that vertebrates arose from tunicate larvae by a process called neoteny. In neoteny, the juvenile form becomes capable of sexual reproduction, and the adult stage is completely bypassed.
Lancelets are very common in shallow water. They are usually hard to see because they bury themselves in the sand, with only the head end sticking out, so they can filter feed by means of the gill slits in their pharynx. As you might expect of a sedentary filter feeder, their cephalization is greatly reduced. Note the segmented musculature in the body. Segmentation evolved independently in the vertebrate line, perhaps as an adaptation for burrowing.
Vertebrates all have a vertebral column or backbone. The linear series of vertebrae, or backbones, reflects the underlying segmentation of the mesodermal tissues. Vertebrate embryos show this segmentation clearly in the muscles that line the back of the embryo. Cephalization is very pronounced, vertebrates are generally active animals. Vertebrates have extremely well developed sensory organs, and a complex central nervous system with a brain encased in a protective skull. Vertebrates have a closed circulatory system, and the sexes are separate. There are seven living classes of vertebrates.
Observe the skeletons and live chordates on display. Pay particular attention to the way in which the critters move, and how that movement is reflected in their skeletal structure. Contrast and compare fish, amphibians, reptiles, and mammals with respect to their method of movement, and the structure of the jaw and mouth. How do these differences relate to their habitat and ecological role (niche)?
Turn your frog over so that the ventral surface is up. Carefully cut through the abdominal wall from between the rear legs to the lower jaw. Cut through the bones of the pectoral girdle as you reach the area of the front legs (try your scissors - they are extremely sharp!). Peel back the skin over the abdomen. Note the thin peritoneal membrane that encloses the large coelom which holds the internal organs. If you have a female frog, much of the coelomic space may be filled with eggs. Carefully remove most of the eggs to reveal the internal organs.
Note the large heart, flanked on each side by a prominent lobe of the liver. Lift up the heart to expose the lungs that lie beneath. Lift up the heart, liver, and lungs to expose the esophagus and the top of the stomach. Note how the esophagus leads up into the pharynx and the mouth. Follow the stomach down to find the small intestine and the large intestine, which leads to the cloaca and the anus. Notice how the intestines are highly coiled to increase surface area for digestion. Cut through one lung to observe its internal structure. What do the architecture of the lung and intestine have in common?
Cut through the stomach and lower intestine, and carefully remove the digestive system to expose the urogenital system. Look for the oval kidneys, lying close to the inner surface of the dorsal body wall. You might see light stripes down the length of each kidney. These are the adrenal glands. Depending on the season in which the frogs were killed, you might also find star-shaped yellow fat bodies, used for fat storage. In which season would these fat bodies be largest? Why? If your frog is a male, you will find two small ovoid testes lying on top of the kidneys. If your frog is a female, you will find two large ovaries in the same position. In both sexes, you will observe a highly coiled oviduct running along the outer edge of each kidney. In the male frog, this oviduct is a vestigial organ.
Stop!! Take a deep breath...You're doing just fine, though maybe just a tad grossed out. But hold on to your chair (or your lab partner), it's about to get a lot worse because...we're going to try to expose the brain!! Turn your frog over, and remove the skin from the top of the head. (Wait! It gets better!!) Now use your surgical scissors to cut through the bones of the skull, starting near the nares (nostrils), just in front of the eyes (ewww...). You will need to very carefully cut and remove the top of the skull in tiny little pieces. It's hard to do this without disturbing the fragile tissues beneath, but give it your best shot....Once the brain is in view, identify the olfactory lobe (smell brain), the cerebrum (thinking brain), and the optic lobe (seeing brain). Notice that the optic lobes are the largest part of the frog's brain? Why? What part of the brain would you expect to be largest in a rat? In a human? How's your brain feeling right now?
Class Agnatha - jawless fish (hagfish, lamprey); 63 sp. (fr. Gr. a = lacking, gnathos = jaw)
These primitive jawless fishes were the very first vertebrates. For about 100 million years, hagfish were the only vertebrates! Their skeletons are composed of cartilage instead of bone. They lack paired fins. All modern agnathans are parasites or scavengers, but their ancestors were filter feeders, probably very similar to the lancelets.
Like the agnathans, these primitive fish have cartilaginous skeletons. But this group shows several key evolutionary advancements, such as jaws to manipulate food. They also have a primitive sensory system called a lateral line, which they share with bony fishes. The lateral line sensors can detect small pressure waves in water, such as those generated by struggling prey. Lateral lines are the fish equivalent of hearing. Their skin is covered with tooth-like structures called denticles.
Shark skin, called shagreen, was once used for sandpaper. Sharks were once an important fishery, and were sought, in the days before synthetic vitamins, for their vitamin-enriched liver (they don't get cancer!). Sharks lack a swim bladder, so when they stop swimming they start to sink. Many sharks will drown unless they are in constant motion, because they can only respire by swimming constantly to force water through the gills. They propel themselves through the water with their powerful tails. The pelvic and pectoral fins are used as horizontal stabilizers or rudders. These paired fins are the humble evolutionary origin of the paired limbs of higher vertebrates.
Bony fishes possess a true bony skeleton, well developed bony jaws, a swim bladder with which they can regulate their buoyancy, and protective scales (note: fish scales are not homologous with the scales of reptiles).
Class Amphibia - frogs, toads, salamanders; 4,200 sp. (fr. Gr. amphi = both, bios = life)
Amphibians were the first animals to emerge onto land, and gave rise to all higher vertebrates. This class dates back about 300 mya, and probably evolved from the lobe-finned fishes like the coelacanth. Their reinforced skeletons enable them to use their pelvic and pectoral bones as limbs to walk about on land. In a very real sense, they never completely left the water. The name amphibian mean amphi=both, bios=life. Amphibians literally they live on both sides of life (land and water).
Amphibians rely on external fertilization in the water. Their eggs are laid directly in the water, to keep them from drying up, and the larvae develop in the water, returning to land as adults. Their lungs, which evolved from the swim bladders of the bony fishes, are relatively weak; they supplement their lungs by breathing through their skin. The amphibians' skin must be kept moist, so terrestrial amphibians are restricted to moist habitats. It also makes them very vulnerable to acid rain, ultraviolet radiation and other aspects of industrial air pollution. Amphibians are vanishing all over the world at a frightening rate.
Class Reptilia may be an artificial grouping, and different groups of reptiles may have independently evolved from different ancestral species. Reptiles are the first fully terrestrial animals, evolving about 280 mya. Unlike amphibians, their limbs hold their bodies off the ground, making for more efficient movement on land. This efficient movement is also aided by better lungs. Rather than sucking air in through their mouths, as amphibians do, reptiles expand and contract the ribs to draw large amounts of air into the lungs, as do birds and mammals. They are covered with scales derived from the epidermis (fish scales develop from the dermis). Scales help keep reptiles from drying out, and are thus an adaptation to terrestrial life. Unlike amphibians, reptiles rely on internal fertilization, another adaptation to life on land. Reptiles possess yet another marvelous terrestrial adaptation, the amniotic egg. The amnion is a protective membrane which forms around the egg following fertilization. Because the developing young are sealed into a shell filled with nutritive fluids, the young can develop entirely on land. This evolutionary innovation is analogous to the seeds of higher plants.
Birds have forearms modified for flight. Their bones are lightweight, and fused together to guard against the stresses and strains of powered flight. The limbs are covered with feathers, structures evolved from scales. Feathers provide insulation and aid in flight. Birds, like mammals, are warm-blooded or endothermic. Birds evolved from theropod dinosaurs in the mid to late Jurassic.
Mammals evolved about 200 mya, and underwent a major radiation during the Cretaceous, literally in the shadows of the dinosaurs. When the dinosaurs vanished, mammals were poised to take their place. Mammals nourish their young with milk from special mammary glands. Although all mammals have nipples, not all mammals have navels. Placental mammals nourish the fetus with in the mother's body by means of a placenta attached to the fetus by a long cord (navels). But many mammals are marsupials, nourishing their young in an external pouch. A few, like the duck-billed platypus and the echidna, are monotremes, mammals that lay eggs like their reptilian ancestors. Like birds, mammals are endothermic. Their bodies are covered with hair, which is a unique evolutionary invention, not related to scales or feathers. Keratin, the same protein that helps form mammalian hair, also forms fingernails, claws, horns, and hooves, in various species of mammals.
Use your imagination! There are literally dozens of ways in which the various groups of chordates are economically important...Think of all the uses of fur and hair and hides (leather). Think about all the chordates that supply food for humans (hamburgers, eggs, fish etc...). What other industries do chordates support or supply?
Why is the evolution of the amniotic egg such an important step? How is it analogous to the evolution of the seed in higher plants?
How does the evolution of segmentation differ in annelids and chordates? Is the ultimate adaptive role of segmentation the same for the ancestors of both groups?
Why are fish bones so lightweight and tiny? Why are the bones of mammals so relatively heavy? Why are bird skeletons the lightest of all?
Why do we consider the swim bladder and lateral fins of primitive fishes to be preadaptations? What evolutionary innovations do they anticipate?
As always, a good place to start is:
And don't forget the dinosaurs:
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