12 - Chordates - Lecture notes 12 PDF

Preview

Summary

Dr Joanne Clarke...

Description

Chordates The phylum Chordata first evolved in the Cambrian period about 530 Mya, it includes Cephalochordates and Urochordata, which don’t have vertebrae. All other chordates are vertebrates (Sub-phylum vertebrata). Derived characters of chordates Notochord - Fluid filled cells encased in tissue which serves as skeletal support. Dorsal Nerve Cord – Hollow tube behind notochord, forms CNS in vertebrates Pharyngeal slits – Vertical slits in the pharynx to strain water and filter food Post-anal tail Many of these are only present in the embryos of higher vertebrates Cephalochordates – Lancelet/Amphioxus, Branchiostoma Free swimming larvae and sedentary adults. They are filter feeders. Urochordates – Tunicates/sea squirts Free swimming larvae and sessile adults. Little change in body plan over 520 my. Evolution of vertebrates 1) Lancelet evolved and tunicates are an offshoot. 2) Tunicate evolved to maintain larval form. More accepted The tunicate genome is very different from that of vertebrates, whereas the lancelets’ are very conserved. Craniates Chordates with a head. 2 sets of hox genes and a neural crest. The neural crest cells give rise to: teeth, parts of skull, inner layer of facial skin, neurons, sensory capsules of ears/eyes. Agnatha – Hagfish and lampreys – Jawless Fish Myxini (Hagfish) - Rudimentary vertebrates, cartilaginous skull but no jaw. Segmented muscle and notochord. Paired eyes, ears, and nasal organs. Slime glands and knotting behaviour. Petromyzidontida (Lamprey) - definite vertebrates, with cartilaginous skulls and vertebrae. Oldest living reps of vertebrates. Marine/freshwater or both. Larvae are filter-feeders and resemble lancelets, adults are often parasitic. Vertebrate Skeleton Initially un-mineralized cartilage, with no nerves or blood supply. Mineralisation into bone occurred after lamprey divergence. Connective tissue is mineralised by osteoblasts with calcium phosphate into bone. Bone might have evolved for mineral storage, to insulate the lateral line system, or to facilitate nonfilter-feeding with the development of teeth. Craniates = chordates with heads Vertebrates = Craniates with backbones...