WEEK 3, BIOLOGY 03051, VERTEBRATE ZOOLOGY: VERTEBRATES; JAWLESS FISHES 2/1/00
Dr. W Crone (303 FTZ, 629-7439, cronewil@hvcc.edu, http://www.hvcc.edu/academ/faculty/crone/index.html)
Text (7th ed.): Ch. 14, pp. 324-329 (especially Fig. 14-19); Ch. 26, pp. 573-581; Ch. 27, pp. 582-597
HVCC library A/V resource: VT 1181,"Conquest of the Waters" (to be seen in recitation)
possible web site: http://www.science.mcmaster.ca/Biology/Harbour/SPECIES/SEALAMP/TITLE.HTM
(an overview of the sea lamprey and its impact in the Great Lakes)
Distinctive features of vertebrates:
As compared to the sea squirts and lancelets from last week, vertebrates have the following distinctive features:3
A way of putting the features of vertebrate dissection into context is to look for common patterns.
Subphylum Vertebrata; the typical vertebrate body segment.1,2 A vertebrate has vertebrae (really!), a body wall, a coelom (body cavity), and internal organs. During development, the embryo has three germ layers of ectoderm (skin and nervous system), mesoderm (connective tissue and muscles), and endoderm (digestive tract) that will set up these adult features during development. Dorsally, mesoderm on either side of the notochord is formed into blocks called somites (which occurs around week 4 in human development1), which help to define the segmentation of the body wall features. Part of the somite forms the vertebral body, which then forms dorsal processes that enclose the dorsal nerve cord in a vertebral arch. Through gaps between the vertebrae run paired spinal nerves run out to the body wall to innervate this striated (skeletal) muscle. Furthermore, bony projections arise off of these dorsal vertebral arches. The transverse processes reach into the myotome to split it into dorsal epaxial musculature and ventral hypaxial musculature (limbs and most of the body wall develop from the hypaxial musculature). The spinous processes offer anchoring for a variety of muscles.
Ventrally, there is unsegmented (splanchnic or visceral) mesoderm surrounding the coelom, with a very different developmental pattern.1,2 Generally speaking, this"coelom-wall" mesoderm gives rise to such things as glands, blood vessels, etc. A dorsal part of this visceral mesoderm, but still ventral to the somites, forms the nephrotome, or beginnings of the genitourinary system. This helps to explain why kidneys are found"deep" to other organs. Strikingly, visceral mesoderm forms smooth muscle and cardiac muscle. These muscle types and glands are not controlled voluntarily by the segmental spinal nerves, but instead by the autonomic nervous system (of sympathetic/parasympathetic fame). The innermost part of the"coelom-wall" mesoderm forms the lining of the coelom, which along the body wall is known as parietal peritoneum and along internal organs as visceral peritoneum. A double fold of peritoneum suspends the gut and contains the gut's blood vessels and nerves--the dorsal mesentery.
The head and neck does not fit this typical segment pattern: e.g., much of the innervation there is taken care of by paired cranial nerves that arise from the brain--human beings have twelve. The pharyngeal arches will contribute to many of the neck (and even some head) structures, and typically different cranial nerves are associated with different arches.
Starting with the fishes, there is an overall pattern of blood circulation as well (think about how this pattern differs in land-dwelling organisms such as ourselves). A ventral aorta takes deoxygenated blood from the heart forward to the gills, where it runs through the gills via capillaries to a dorsal aorta. This oxygenated blood is then directed to the head and to the rest of the body. The intestines are drained by a (hepatic) portal vein to the liver, and a hepatic vein drains the liver. Cardinal veins drain the body wall and muscles. Veins drain into the sinus venosus of the heart, to the atrium, to the ventricle, and out of the conus arteriosus to repeat the cycle. As a result of all the above, keep an eye out in lab for this generic pattern and try to pick out the variations in the different organisms that you're dissecting.
Generally, we use the term"fish" to describe those vertebrates with fins and gills, even though this encompasses a wide variety of organisms (arguably wider than that of the land-dwelling vertebrates). Living in the water has dictated many characteristics, e.g., overall body form, method of respiration, eating, etc. The first vertebrates were jawless filter feeders, who used pharyngeal pumping to draw fluid in.
Ostracoderms ("shell skins") were heavily armored with dermal bone formed underneath skin (vs. cartilage-replacement bone inside), presumably to protect against predators. Their main internal support was the notochord.4 Many of these ostracoderms were finless. Today, jawless fish include the lampreys and hagfishes.
Lampreys (Class Cephalaspidomorphi"head shield shape"): no lateral fins, scaleless. The"teeth" in the funnel are for rasping, but are epidermal in origin and not like real teeth, which are mesodermal. Lampreys can be diadromous; they come to fresh water to spawn. Sea lampreys can live in both salt and fresh water, and adults live by attaching to fish and sucking blood and body fluids through the hole they rasp in the side. The ammocoetes (larval form of the lamprey) looks very similar to the amphioxus, but with internal vertebrate features. These larvae spawn in streams and live in mud burrows until metamorphosis into the parasitic adults.
Structurally, lampreys barely live up to the term"vertebrate"--their small, cartilaginous vertebral elements are dwarfed by the notochord.4 Their digestive tract does not show the major distinction of a separate stomach (can you think why?), but has a fold or typhlosole in the intestine for more surface area.
Hagfishes (Class Myxini"slime"): marine, bottom-dwelling, apparently no separate larval stage. Many mucous glands, hunts by olfaction (smell). Eats by slipping a knot in its body forward to push chunks of meat off of dead and dying fish.3,4 This has earned the dislike of fishermen, but hagfish skin is the basis for"eelskin wallets" (which is leading to hagfish overfishing).4
Hagfish are even more unusual than lampreys internally, and as shown by the cladogram on Fig. 27-2, are considered separate from other vertebrates. Such features as no vertebrae at all, accessory hearts (e.g., towards the tail), only one semicircular canal in their ears (we have three), and primitive kidneys that keep hagfish internal fluids at the same salt level as the sea around them all emphasize their uniqueness.3,4
Development of jaws. Jaws are useful for biting and feeding. Vertebrate jaws are a modification of the first gill arch (so-called mandibular arch). If we follow along in vertebrate embryos, we will see that these initial cartilages get covered by secondary dermal jaws. In land animals, the upper jaws are firmly attached to the skull, but in most fishes, the jaws can both be moved (supported by gill arch II, the hyoid arch).1,2,4
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