Brian McCauley & Bruce Heyer 12/12/07 Page 1 of 13 Introduction to Chordates & Fish Anatomy Biology 6A / Sanhita Datta Winter 2008 Today’s lab has two parts: • Introduction to Chordates: an introduction to the key characteristics of the phylum Chordata, with just a few specimens to study. • Fish anatomy: dissection of some fresh fish in order to learn their external and internal anatomy. The next two labs will also be dedicated to this phylum: one lab for mammalian anatomy as seen in fetal pigs, and one lab for vertebrate skeletons. Chordate characteristics TISSUES: • Three well-defined tissue layers in embryo. SYMMETRY: • Bilateral, with cephalization. BODY CAVITY: • Coelom PROTO/DEUTEROSTOME: • Deuterostome: the blastopore formed during gastrulation eventually becomes the anus; the mouth forms later. DIGESTIVE TRACT: • Complete digestive tract. CIRCULATORY SYSTEM • Closed in vertabrates; open in a few others. OTHER FEATURES • Segmented body. Vertebrae, for example. • Endoskeleton • Notochord: a connective-tissue body stiffener • Dorsal tubular nerve cord – forms brain and spinal cord. • Pharyngeal pouches and slits – gill related structures that may appear and then disappear early in development. • Postanal tail. In many worms, the anus is at the very tip end of the animal’s body; chordates typically have a tail beyond the anus.Brian McCauley & Bruce Heyer 12/12/07 Page 2 of 13 Early development of chordates The defining features of chordates appear early in development. Gastrulation Like other animals, chordates progress through blastula and gastrula stages. Gastrulation not only forms the beginning of the digestive tract, it also forms the three embryonic tissue types: endoderm, mesoderm, and ectoderm. The diagram at right shows gastrulation in a frog. The process is somewhat similar to gastrulation in echino-derms (as shown in last week’s handout on animal tissues and development). However, chordate gastrulation is somewhat more complex, partly because the early embryo is more asymmetrical and more filled in with cells. Chordates are deutero-stomes. This means that the blastopore (the opening into the archenteron that forms during gastrulation) becomes the anus; the mouth forms later. Echinoderms are also deuterostomes, but annelids and arthropods are protostomes. In those phyla, the blastopore becomes the mouth, and the anus forms later. This is one of the reasons that chordates are considered to be more closely related to echinoderms than to arthropods. Notochord formation The notochord, one of the unique defining characteristics of chordates, is a semi-stiff rod of connecting tissue that forms in the embryo and to guide the development of the vertebral column (backbone) of vertebrates, along with other structures. In your own body, the notochord has mostly disappeared; the only remnants are the cartilaginous disks between your vertebrae Neurulation Neurulation forms the dorsal hollow nerve cord in a developmental event that bears some resemblance to gastrulation. The dorsal hollow nerve cord is another unique feature of chordates. Following gastrulation, the presence of a notochord induces neurulation of the overlying dorsal ectoderm. This third stage of morphogenesis is unique to chordates. The ectoderm above the notochord thickens to form the neural plate. This plate then invaginates to form a furrow along the anterior-posterior axis. The folds along the groove eventually seal over the furrow to form theBrian McCauley & Bruce Heyer 12/12/07 Page 3 of 13 neural tube that in turn develops into the dorsal hollow nerve cord. This nerve cord eventually develops into the central nervous system, including the spinal cord and brain. By contrast, in non-chordate animals the main nerve cord is solid and usually ventral. The diagram below (fig. 47.14 from Campbell) shows neurulation and associated events in a frog. Chordate development specimens:  Models of frog development & neurulation. Note the stages of development following gastrulation. Identify the subsequent appearance of the notochord, neural plate, and finally the neural tube and neural crest. Compare to Campbell Fig. 47.14.  Slides of 13-hour & 18-hour developing chick embryos. Compare the developing chick to the models of frog morphogenesis and identify the corresponding homologous structures named above. Refer to the Photo Atlas Fig. 2.16. ☛ (CAUTION: We will be examining today several slides of this series showing stages of development in the chick embryo. The slides with the later stages are thick and can only be viewed at low power. Don’t break them by using a higher power objective.) Non-vertebrate chordates: urochordates and cephalochordates Urochordates and cephalochordates are in the phylum Chordata, but they aren’t vertebrates; in fact, you might not immediately recognize them as belonging to your own phylum. Urochordates (tunicates) Tunicates (also called sea squirts) are marine suspension feeders. They live in the ocean, pump water though their gut, and capture small particles of food suspended in the water. They are different from vertebrates in many respects: No cephalization of the dorsal nerve tube – in other words, no brain. Open circulatory system that can even reverse its direction of flow.Brian McCauley & Bruce Heyer 12/12/07 Page 4 of 13 Pharyngeal arches and slits form a ciliated filter basket (pharynx) used for gas exchange and suspension feeding. Water is drawn in through an incurrent siphon and pumped out through an excurrent siphon, passing through the slits in the pharynx. A sheet of mucus is used to capture suspended particles. The arches of the pharynx are vascularized, and serve for gas exchange as well as food capture. Many urochordates are sessile; they spend their adult lives glued to the bottom of the ocean. The diagram above (fig.34.4 from Campbell shows adult tunicates in (a) and (b), and a larval tunicate in (c). While urochordates differ from vertebrates in many ways, they also show the defining characteristics of the phylum Chordata, including the notochord,