BLY 202 Superclass Agnatha: Jawless Fishes Objec'ves are to • Gain understanding of the characteris'cs of the jawless fishes • Have knowledge of the forms and func'ons of the two classes of the Superclass Agnatha • • Compare and contrast between Class Myxini and Class Cephalaspidomorphi Introduc'on • Living jawless fishes are represented by approximately 84 species • Myxini (hagfishes) with about 43 species • Cephalaspidomorphi (lampreys) with 41 species • Members of both groups lack jaws, internal ossificaon, scales, and paired fins, and both groups share pore-like gill openings and an eel-like body form • In other respects, the two groups are morphologically very different Characteriscs of Class Myxini • Body slender, eel-like, rounded, with naked skin containing slime glands • No paired appendages, no dorsal fin • Fibrous and car'laginous skeleton; notochord persistent • Bi'ng mouth with two rows of eversible teeth • Heart with sinus venosus, atrium, and ventricle; accessory hearts, aor'c arches in gill region • Five to 16 pairs of gills with a variable number of gill openings Characteriscs of Class Myxini cont… • Segmented mesonephric kidney; marine, body fluids isosmo'c with seawater • Diges've system without stomach; no spiral valve or cilia in intes'nal tract • Dorsal nerve cord with differen'ated brain; no cerebellum; 10 pairs of cranial nerves; dorsal and ventral nerve roots united • Sense organs of taste, smell, and hearing; eyes degenerate • Sexes separate (ovaries and testes in same individual but only one is func'onal); external fer'lizaon; large yolky eggs, no larval stage Class Myxini: Hagfishes • Hagfishes are an en'rely marine group that feeds on annelids, molluscs, crustaceans, and dead or dying fishes • They are not parasi'c like lampreys but are scavengers and predators • The best known in North America are the Atlan'c hagfish Myxine glunosa and the Pacific hagfish Eptatretus stou+ • Almost completely blind, hagfish is quickly aracted to food, especially dead or dying fishes, by its developed senses of smell and touch • The hagfish enters a dead or dying animal through an orifice or by digging inside. Using two toothed, keranized plates on the tongue that fold together in a pincer-like ac'on • The hagfish rasps away bits of flesh from its prey Class Myxini: Hagfishes cont… • Hagfishes are renowned for their ability to generate enormous quan''es of slime • If disturbed or roughly handled, the hagfish exudes a milky fluid from special glands posi'oned along the body • Unlike any other vertebrate, the body fluids of hagfishes are in osmo'c equilibrium with seawater, as in most marine invertebrates • Hagfishes have a low pressure circulatory system served by 3 accessory hearts in addi'on to the main heart posi'oned behind the gills • There is no larval stage Characteriscs of Class Cephalaspidomorphi • Body slender, eel-like, rounded with naked skin • One or two median fins, no paired appendages • Fibrous and car'laginous skeleton; notochord persistent • Sucker like oral disc and tongue with well-developed keranized teeth • Heart with sinus venosus, atrium, and ventricle; aor'c arches in gill region • Seven pairs of gills each with external gill opening Characteriscs of Class Cephalaspidomorphi cont… • Opisthonephric kidney; anadromous and fresh water; body fluids osmo'cally and ionically regulated • Dorsal nerve cord with differen'ated brain, small cerebellum present; 10 pairs cranial nerves; dorsal and ventral nerve roots separated • Diges've system without stomach; intes'ne with spiral fold • Sense organs of taste, smell, hearing; eyes well developed in adult; two pairs semicircular canals • Sexes separate; single gonad without duct; external fer'lizaon; long larval stage (ammocoete) Class Cephalaspidomorphi (Petromyzontes): Lampreys • All the lampreys of the Northern Hemisphere belong to the family Petromyzon'dae • The group name refers to the lamprey’s habit of grasping a stone with its mouth to hold posi'on in a current • There are 22 species of lampreys in North America • About half of these belong to the – nonparasi'c type; – the others are parasi'c • The genus Ichthyomyzon, includes three parasi'c and three nonparasi'c species. Class Cephalaspidomorphi (Petromyzontes): Lampreys cont… • All lampreys ascend freshwater streams to breed • The marine forms are anadromous, that is, they leave the sea where they spend their adult lives to swim up streams to spawn • At spawning, with the female aached to a rock to maintain her posi'on over the nest, the male aaches to the dorsal side of her head. As eggs are shed into the nest, the male fer'lizes them • The adults die soon aer spawning. The eggs hatch in about 2 weeks, releasing small larvae (ammocoetes) • Parasi'c lampreys either migrate to the sea, if marine • Or remain in fresh water, where they aach themselves by their sucker-like mouth to a fish and, with their sharp keranized teeth, rasp away the flesh and suck out body fluids. Class Cephalaspidomorphi (Petromyzontes): Lampreys cont • To promote the flow of blood, the lamprey injects an an'coagulant into the wound • When gorged, the lamprey releases its hold but leaves the fish with a large, gaping wound that is some'mes fatal • The parasi'c freshwater adults live 1 to 2 years before spawning and then die • The anadromous forms live 2 to 3 years • Nonparasi'c lampreys do not feed aer emerging as adults and their alimentary canal degenerates to a nonfunc'onal strand of 'ssue • Within a few months they also spawn and die. .