Evolutionary Adaptations in Agnatha

Superclass Agnatha

• Comprised of jawless fish, including Hagfish (Myxine) and Lamprey (Petromyzon marinus).
• Five to sixteen gills are used for respiration in Myxini; Cephalospidomorphi have seven pairs with external openings.
• Lacks a stomach; features a dorsal nerve cord and differentiated brain.
• External fertilization occurs with no larval stage; sexes are separate.
• Adaptations include long, slender, limbless body and slimy skin for reduced water resistance.
• Tail with caudal fin enhances forward thrust; buccal funnel and toothed tongue adapted for blood-sucking.

Superclass Gnathostomata

• Comprised of six classes: Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves, and Mammalia.

Class Chondrichthyes

• Known as cartilaginous fishes, including sharks, rays, and skates.
• Possess jaws evolved from the first pair of gill arches; skeletons are largely cartilaginous.
• Body shape is laterally compressed and fusiform; mouth is located ventrally.

Class Amphibia

• Body forms vary, with a clear division into head and trunk.
• Most have two pairs of pentadactyl limbs; some, like caecilians, lack legs.
• Skin is smooth, moist, and highly vascular, often with glandular structures.
• Respiration in larval stage is via gills, whereas adults use lungs and skin.
• Heart has three chambers; undergoes double circulation.
• Fertilization can be external or internal; most are oviparous with a metamorphic life cycle.
• Temperature regulation is poikilothermic, leading to behaviors such as hibernation.

Evolutionary Adaptations in Amphibia

• Significant transition from aquatic to terrestrial life.
• Adaptations include limbs for movement, lungs for air breathing, and internal nares for closed-mouth breathing.
• Slimy skin helps prevent desiccation.
• Circulatory system adapted for lung and skin respiration; reduced skeletal mass for a lighter body.

Classification of Mammals

• Divided into three subclasses: Prototheria (egg-laying), Metatheria (pouched), and Eutheria (placental).
• Prototheria exemplifies a link between reptiles and mammals; features include cloaca and mammary glands.
• The Duckbill platypus and Echidna are key examples of monotremes.

Subclass Metatheria - Marsupials

• Females possess a pouch (marsupium) for immature young.
• Young are nurtured in the pouch until maturing; significant examples include Opossum, Kangaroo, and Tasmanian wolf.

Sponges

• Totally lack a nervous system; composed of various spicule shapes forming a skeletal framework.
• Involuntary digestion occurs intracellularly within choanocytes, with respiration facilitated through diffusion and water currents.
• Reproduction is both sexual (egg and sperm) and asexual (budding or gemmules).
• Known for exceptional regenerative capabilities; evolved from protists called choanoflagellates.
• Economic uses include personal hygiene, medical procedures, and architectural soundproofing.

General Features of Animals and Classification Criteria

• Distinct features: multicellular, heterotrophic metabolism, specialized tissues, mobility.
• Criteria for classification generally encompass anatomical, physiological, and genetic characteristics.
• Radial symmetry adapts organisms to a sessile or free-floating lifestyle; bilateral symmetry allows for directional movement.

Responses to Questions

• Amphibians arose from jawless fish.
• Deuterostome characteristics include blastopore associated with the anus, enterocoelom, and inclusion of echinoderms and chordates.
• Gastrovascular cavities are found in flatworms.
• Non-chordate subphylum: Hemichordata.
• Ectothermic animals (like fish) regulate body temperature externally; endothermic (like mammals) regulate internally.

This quiz delves into the unique characteristics and evolutionary adaptations of the agnatha superclass. Discover the differences in gill structures and reproductive methods of classes Myxini and Cephalospidomorphi, along with their notable examples like hagfish and lamprey. Test your knowledge of these fascinating jawless fish.