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WEEK 2, VERTEBRATE ZOOLOGY 03051: PHYLUM CHORDATA;

SUBPHYLA UROCHORDATA AND CEPHALOCHORDATA

1/24/00 Text (7th ed.): Ch. 15; Ch. 26.

Dr. W. Crone (303 FTZ, 629-7439, cronewil@hvcc.edu, http://www.hvcc.edu/academ/faculty/crone/index.html)

possible web site: http://www.ucmp.berkeley.edu/chordata/chordata.html (chordate overview)

1. Taxonomy, or science of classification, based on the two main concepts of Linnaeus: binomial species name and hierarchical groupings. Classification is our attempt to show the relatedness of organisms, and hence, should reflect evolutionary relationships."King Philip, Come Out For Goodness Sake:" Kingdom, phylum, class, order, family, genus, species. In addition to these major categories, biologists also use subphylum, superclass, etc. In Zoology 03051, we will learn about:

Phylum Chordata

  1. Subphylum Urochordata: tunicates or sea squirts (this week)
  2. Subphylum Cephalochordata: lancelet or amphioxus (this week)
  3. Subphylum Vertebrata: vertebrates (rest of the semester!)

Classes of vertebrates: (currently living)

  1. Cephalaspidomorphi: lampreys
  2. Myxini: hagfish
  3. Chondrichthyes: cartilaginous fish, i.e., sharks and rays
  4. Osteichthyes: bony fish
  5. Amphibia: amphibians
  6. Reptilia: reptiles
  7. Aves: birds
  8. Mammalia: mammals

2. These familiar evolutionary groupings have been challenged in recent years by phylogenetic systematics or cladistics. The"branch diagrams" of cladistics (e.g., Fig 29-2, p. 624) are attempts to indicate ancestral vs. derived characteristics, and the"branch points" or common ancestries of different groups of animals."Fish gave rise to amphibians, which gave rise to reptiles, which gave rise to birds and mammals" is a shorthand expression that can lead to confusion. How can a modern-day frog be descended from a modern-day fish? But, if we think about common ancestors of specific vertebrate groups, then we should be able to appreciate what cladistic diagrams are trying to describe.

3. Phylum Chordata (L."cord"): major characteristics. Chordates are bilaterally symmetric, deuterostome animals (refers to patterns of development, a characteristic shared by echinoderms), with a true body coelom (body cavity that separates the body wall from internal organs). A chordate exhibits somewhere in its life cycle the following four unique structures:

  1. notochord: the early endoskeleton (internal skeleton), a flexible, dorsal rod that runs the length of the animal. Sharks replace its support function with cartilage, (most) other vertebrates with bone.
  2. dorsal, tubular (hollow) nerve cord. This tubular nerve cord runs dorsal to the notochord, and is generally expanded in chordates anteriorly into a brain.
  3. pharyngeal pouches and slits. The pharyx is the region posterior to the mouth. Here are often found narrow openings from the digestive tract to the outside called gill slits. Chordates may also show outpocketings or pharyngeal pouches. These features are best seen in gilled fish; human pharyngeal pouches are embryonic features.
  4. postanal tail that extends posterior to the anal opening.

In addition, many chordates will typically have other noticeable features, based on metamerism or segmentation of body parts; especially seen in the spine, musculature, and location of blood vessels and nerves supplying the body wall (to be described next week).

Given these features, which group of animals could be ancestral to chordates? Surprisingly, the echinoderms, with their similar developmental patterns and with some interesting fossil groups, may be among the invertebrates with whom we most recently shared a common ancestor.1

4. An overall appreciation of major organ systems will assist in our understanding of the semester's organisms.

  1. integumentary system (skin)
  2. musculoskeletal system (endoskeleton and voluntary skeletal muscles)
  3. cardiovascular system (heart, artery, capillary, vein)
  4. respiratory system (lung, gill)
  5. gastroinestinal system (mouth, esophagus, stomach, intestine, anus, liver, gall bladder, pancreas)
  6. urinary system (kidney, ureter, bladder, urethra)
  7. endocrine system (hypothalamus, pituitary, thyroid, adrenal, [gonad, pancreas])
  8. immune system (spleen, tonsil, thymus, lymph node)
  9. reproductive system (gonad, oviduct, vas deferens, uterus, copulatory organ)

5. Subphylum Urochordata ("tail cord"): the tunicates or sea squirts.

5a. Why they are chordates: The larval tunicates contain the major characteristics of the phylum: i.e., notochord, dorsal nerve cord, postanal tail, and pharyngeal gill slits. These larvae swim around in tadpole fashion in the ocean for a short time before metamorphosis (change in form) into the sessile (attached, not moving) adult form.

5b. What happens during metamorphosis: the tail is absorbed, other larval structures disappear, and epidermal cells secrete a tunic or covering, which at its anterior end contains adhesive papillae, so that the adult is oriented in a"headstand."

5c. Features of the adult tunicate. An incurrent siphon draws food-containing water into the pharynx, through the pharyngeal gill slits, and then out an excurrent siphon. The food is collected by mucus secreted by the endostyle (equivalent to our thyroid), to be digested in the rest of the digestive tract. A heart has the unusual feature of driving the blood in one direction, pausing, and then moving it in the other.1 Most tunicates are hermaphroditic (containing both male and female organs), dumping gametes into the water for cross fertilization.

6. Subphylum Cephalochordata ("head cord"): lancelet or amphioxus.

6a. Why the amphioxus (lancelet) is a chordate. Branchiostoma is a small 5-7 cm), transparent, marine, fish-like organism that spends most of its time buried in the sand filter feeding. It has a persistent notochord; the dorsal nerve cord dorsal to the notochord; pharyngeal gill slits; and postanal tail. Furthermore, the muscles are arranged in visible segments (myomeres). Unlike the sea squirt, the lancelet larve is reasonably similar to the adult.

6b. Aspects of the adult amphioxus. Filter feeding occurs by water passing through the mouth after being stirred up by cilia (microscopic hair-like structures) on the gill bars that separate the pharyngeal slits. Cirri or tentacle-like structures under the oral hood catch the larger particles, but the smaller particles are collected by the gill bar cilia and by mucus (from the endostyle). The food is thrown off of the mucus conveyor belt and into the rest of the digestive tract. A small pouch or midgut cecum secretes digestive enzymes along the way before the final exit through the anus. Food is moved by cilia, not the muscular contraction or peristalsis familiar to us (hence no need for nervous system wiring to the gut, as we see in ourselves). While heartless (a ventral aorta does the pumping), the overall pattern of the lancelet circulation is similar to that of the vertebrates. There are separate male and female lancelets, but fertilization is still external. Although an adult amphioxus lacks a brain and many sensory structures (and hence, probably isn't the most likely candidate for our direct ancestor), it still gives a reasonable idea of what early, prevertebral chordates may have looked like.

 

  1. CP Hickman et al., Biology of Animals, 7th ed. (WCB McGraw-Hill, Boston, 1998), pp. 570, 571.


|main page| |background| |03028: Physiology| |03048: Anatomy|

|03050: Invertebrate Zoology| |03051: Vertebrate Zoology| |03074: Economic Botany|


Please send comments and questions to: cronewil@hvcc.edu

 

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