BIOL 2015 – Evolution and Diversity Lab 11: Deuterostomia: Echinodermata, Hemichordata, & Chordata

Introduction

In our final diversity lab we will examine the deuterostomes, which include the following three phyla: Echinodermata, Hemichordata, and Chordata. Although these phyla seem outwardly very different, they share common features in their early development that distinguish them from other bilaterally symmetrical animals. Remember in deuterostomes, the embryonic blastopore gives rise to the anus. In the protostomes, which includes annelids, arthropods, and mollusks, the blastopore gives rise to the mouth. Although adult echinoderms have radial symmetry, their larvae are bilaterally symmetrical, thus they are classified along with chordates as bilaterally symmetrical.

Figure 1: Phylogeny of Animalia

Echinodermata

The word echinoderm means spiny skin. These are marine organisms with five-fold radial symmetry, commonly called starfish (Class Asteroidea), sand dollars and sea urchins (Class Echinoidea), sea cucumbers (Class Holothuroidea), and sea lilies (Class Crinoidea). They are coelomates and they have an endoskeleton that is made of calcium carbonate, which arises from the mesoderm. Sexes are separate in echinoderms and fertilization is external; there is a larval stage with bilateral symmetry. A unique derived feature of the group is its water- vascular system; seawater is pumped through a series of ducts to work the tube feet and the suction cups at their tips.

Asteroidea (Starfish) Find the preserved starfish on display. On the dorsal side you can see the central disk and radiating arms. Just off center on the central disk there is a small sieve-like plate, called the madreporite, which serves as the conduit through which seawater enters into the water vascular system. Small fleshy extensions among the spines are the soft, hollow skin gills for respiration; they communicate with the coelom. On the ventral side of the arms there are ambulacral grooves that are filled with the fleshy tube feet. Tube feet and skin gills Figure 2: Starfish are both used for gas exchange and excretion of nitrogenous waste. The mouth is in the center of the ventral side. Echinoderms move by alternating the suction and release of tube feet. Asteroids can regenerate lost or damaged arms, if enough of the central disc is intact. An isolated arm soon dies (there is an exception, known in one genus where an arm can regenerate the rest). Use a dissecting microscope to find the pincer-like pedicellariae that project from the surface of the dermis. Examine an opened starfish and note the large coelom. Find the ampullae at the bases of the tube feet, and the ring and radial canals of the water vascular system. Observe the living starfish in the aquarium and notice the movements of their tube feet.

Ophiuroidea (brittle stars) Find the preserved brittle stars on display. Organs are concentrated in the large central disk because they have long slender arms. Brittle stars primarily use their arms to move with speed and agility. Some brittle stars are carnivores while others are filter feeders.

Examine the dorsal surface and notice the absence of a madreporite. On the ventral surface, notice that ossicles cover the ambulacral grooves and that the mouth is surrounded by five moveable jaw plates. The bursal slits (which radiate around the mouth) open into sacs in which water constantly circulates, and gas exchange occurs.

Find the living brittle stars in the display large aquarium and note where they reside within the aquarium.

Echinoidea (Sea Urchins, Sand Dollars) Echinoids have a shell made from well-developed dermal ossicles. The Tube feet line the ambulacral regions, but the ambulacral grooves are closed. Pedicellariae are present, like starfish. Sea urchins move slowly across the substrate consuming small animals or scraping algae off of rocks with powerful, plate-lined jaws. Observe

the living sea urchins in Room the large tank and notice how the spines move (muscles at the spine’s base controls their movement).

Examine the preserved sea urchins on display.

Examine the preserved sand dollars on display. Note how the shell is flattened. Be able to identify the different echinoids.

Holothuroidea (Sea Cucumbers) Although they are echinoderms, sea cucumbers have soft skin than other echinoderms. All that remains of their calcareous skeletons are small spicules. Their ambulacral regions extend from mouth to anus, and the ambulacral grooves are closed. Modified tube feet called circumoral tentacles surround the mouth. The pedicellariae seen in other echinoderms are absent. Find the living sea cucumbers in the main aquarium in Room 170, and observe the circumoral tentacles.

Examine the preserved sea cucumbers on display.

Hemichordata

Hemichordates are small to medium sized marine worms and are represented by three extant classes, Enteropneusta (acorn worms), Pterobranchia (small, deep water worms), and Planctosphaeroidea (known only by larva of one species). Enteropneusta is the most specious of these three classes and is represented by about 70 species. This will be the only Hemichordate we have for you to examine in lab.

Enteropneusta (acorn worms) Of the chordate characteristics, acorn worms have only pharyngeal slits. They feed on detritus and burrow in soft marine sediments.

Find the preserved acorn worm on display and observe (be gentile – they are fragile) the proboscis, the collar, the trunk and the pharyngeal slits.

Figure 3: Acorn Worm

Chordates

The Phylum Chordata contains three subphyla, Cephalochordata, Urochordata, and Vertebrata (sometimes called Craniata). We will look at the Cephalochordata, the lancelets. Another Subphylum is the Vertebrata; Fish, amphibians, reptiles, birds, and mammals are all vertebrates. Amphioxus is most likely the descendant from an early stage in chordate evolution, and it shows all five of the defining characteristics of chordates:

1. notochord 2. pharyngeal slits 3. endostyle or thyroid gland 4. dorsal hollow nerve cord 5. post-anal tail

All chordates show these characteristics at some stage in their lifetimes. You will also notice features that are common among many animal phyla, including bilateral symmetry, coelom, and body segmentation (seen in the series of muscle blocks called myomeres). The circulatory and digestive systems are separate.

The first two subphyla, Cephalochordata and Urochordata, are often referred to collectively as the Protochordates, meaning the first Chordates. While all 5 synapomorphies are present in these two subphyla, some persist throughout the entire lifespan of the organism while others are transient and are only present in the larval state.

Cephalochordata (Amphioxus)

Although they spend most of their time buried in the sand filter feeding on small food particles, lancelets are small, 'fish-like' animals and are decent swimmers.

Find the preserved Amphioxus on display. There is relatively little cephalization in lancelets, but you may observe a small enlargement at the anterior end of the dorsal nerve chord. Like vertebrates, but unlike tunicate larvae, the trunk muscles have myomeres (segmented structures). The endostyle (a ciliated groove in the pharynx) is used to trap food particles and transport them to the stomach. Also observe the notochord and the atriopore, which is the equivalent of the excurrent siphon of tunicates, and the anus and post anal tail.

Find a prepared slide labeled "Amphioxus immature w.m." and "Amphioxus immature adult w.m."

Figure 4: Lancelet

Find all the chordate features on the immature slide (you may not be able to see the endostyle). The adult slide shows the pharyngeal slits and tentacles (cirri) better. You should also be able to locate the forward- pointing cecum of the digestive tract. What does the cecum do?

Find a prepared microscope slide labeled "Amphioxus Pharynx and Gonad sec."

Find the notochord, dorsal nerve cord, pharynx with gill bars and gill slits, and the large muscles. You should try to find the endostyle at the bottom of the pharynx, and try to distinguish the atrium from the coelom.

Figure 5: Lancelet

Urochordata (Tunicates)

Ascidiacea Although urochordates may look a bit like sponges in their adult life stage, they are currently thought to be the sister group of vertebrates. Why might these be the better sister group to Vertebrates? Most Urochordates are hermaphroditic and use cross-fertilization to reproduce. Some have a single testis and single ovary (when both gonads are combined into a single mass it is often called the ovotestis) and are located in the loop of the gut. Some species possess many gonads that are embedded in the connective tissue layer of the body wall. Solitary species are oviparous and their eggs have very little yolk. Eggs are released through the atrial (excurrent) siphon and are fertilized externally. In contrast, most colonial species are viviparous and their eggs have significant amount of yolk. Gestation takes place in either the oviduct or within the atrium itself and the lecithotrophic tadpole larvae are then released from the atrial siphon.

Find the preserved sea squirts on display (Ciona and Molgula). Observe the oral (incurrent) and atrial (excurrent) siphons in Ciona (you don’t need to distinguish them in Molgula).

Find a prepared microscope slide labeled "Ecteinascidia w.m."

Use the 4X objective to observe these adult animals. Find the siphons, the very large pharynx with its conspicuous perforations.

The Ascidian tadpole larvae are lecithotrophic and therefore obtain all their nutrients from the yolk. So, depending on the amount of yolk, the larval stage can be extremely short ranging from a few minutes to about 36 hours. It is during this stage that all five Chordate characters are evident. Some of the Chordate characters are lost as the tadpole undergoes metamorphosis and transforms into the sessile filter-feeding adult that we are familiar with.

Find a prepared microscope slide labeled "Ascidian Tadpole Figure 6: Ciona w.m."

The tatpoles are small and it is difficult to identify many of the internal structures. Find the post anal tail and the notochord.

Vertebrata

Vertebrata is the Subphylum that we belong to as mammals and it includes the fishes (chondrichthyes and osteichthyes), amphibians, reptiles, and birds. Unfortunately we do not have enough time to really explore the diversity of the Vertebrates, which is the most specious group within Deuterostomia. Fortunately, vertebrates are usually more familiar to the vast majority of people and are therefore much easier to learn and remember. If you are interested in learning more about the vertebrates you should consider taking BIOL 3315, Comparative Vertebrate Morphology Laboratory.