In lieu of an abstract, here is a brief excerpt of the content:

PERSPECTIVES IN BIOLOGY AND MEDICINE Volume 25 · Number 2 ¦ Winter 1 982 OPERA CHORDATA THOMAS B. ROOS* The phylum Chordata has many beasts in it, From pipefish to sea-squirt, from wombat to linnet. What possible features could all have in common To set them apart with a special cognomen? A backbone, not likely, Nor braincase nor bone, None's found in a tunicate, Live or in stone. What then can be named to encompass this band Of creatures so common on sea and on land? Chorus A notochord, nerve tube, and tail, Pharyngeal pouches and veil. Each may be changed beyond all recognition While embryos grow or aform's in transition. But all will appear withoutfail: Veil, pouches, 'chord, nerve tube and tail. Urochordata have larvae with tails, A pharynx that seems big enough for a whale's. Their tadpoles swim actively, cavort and play, But settle to sedentate after one day. Their nerve cord, don't doubt it They keep in their tail. Their notochord, also Where it's doomed to fail. For when they have settled and metamorphosed ?Department of Biological Sciences, Dartmouth College, Hanover, New Hampshire 03755. Permission to reprint may be obtained only from the author. Perspectives in Biology and Medicine, 25, 2 · Winter 1982 \ 171 Their tail has resorbed and their mouth has transposed. Another tail-chordate swims round through the ocean, Its body all flat like a ribbon in motion. The group as a whole hasn't much to commend it, Although it's from whence all of us have descended. The lancelets, included, With pulsating hearts, A head with a chorda, A brain without parts, These miniscule fishlets with mouths ringed by pegs, Cephalochordata, sans bone and sans legs. Repeat Chorus True vertebrates make up their very own phylum, A large one with species both living and whilom. They all have vertebrae, 'though not all are bony, The fishes have gill slits, some true and some phony. Tetrapode, mostly, can Walk on their hindlimbs Though in snakes and in whales You can't hardly find limbs. Agnatha, the jawless, abjure bone and limb, While living as parasites, silent and grim. Oh lamprey and hagfish, what dismal descendants Of Osteostraci whose head shield's resplendence, Resisted all efforts of monstrous arachnids, Silurian titans whose mandibles crack'd lids. Pterapsids, with wings, that Were useless for flight, AU one nostrilled creatures With pineal sight. Remarkable agnaths, most ancient of fish Whose anadromosity ends on a dish. Repeat Chorus Another old class with degenerate scions, The sharks, rays, and skates live throughout the oceans. Their gill slits all open quite free of each other, And many are borne fully grown from their mother. Chimera, a cousin, 1 72 I Thomas B. Roos ¦ Opera Chordata Lives deep in the sea, With covered up gill slits And its skin scale-free, While shark skin is covered with enameled scales: Big teeth onjaws, and small ones on tails. Elasmobranchs ancient had bones in and outside, Their jaws were enormous, adept at piscicide Arthrodires raided Devonian fisheries, Jaws opened upward to seek out habitués. Deep waters they haunted, And shallow seas too. But kept out of fresh streams Where ions were few. But now the old skeleton's gone for an age, Replaced by a new one of pure cartilage. Repeat Chorus The rest of the fishes, of class Osteichthyes, Have bony scales unlike the toothy Dinichthyes. Extinct Acanthodians, while in their heyday, Had multiple fins both for working and foreplay. Some had twelve, some had less But most more than four, The usual number Of limbs evermore. Most Sarcopterygii also have gone, Though numbering tetrapode amongst their spawn. Most fish that have bones in this bounteous era The source of trout fillets and cod in Madeira, The sturgeons whose eggs feed the princes of Iran The herring of Holland, the carp out in Yunan. All of them, it is true, Whatever their ways Have thin fins supported By delicate rays. Lungs in the old forms and air sac in later, Raise them and sink them, a self-elevator. Repeat Chorus Choanate sarcopts with muscular shoulderjoints, Powerful forearms and fingers with claw-tip'd points. Perspectives inBiohgy andMedicine...

pdf

Share