Vertebrates And The Head

The vertebrates, the major group of chordates, form the subject of this book. They have sometimes been termed craniates since all forms, including the hag-

fishes and lampreys, have specialized head features (the cranium, the skull). The term vertebrate is better known, so will be used here, following recommendations by Donoghue etal. (1998).

The basic vertebrate body plan (Figure 1.9) shows all of the chordate characters so far described — notochord, dorsal nerve cord, pharyngeal 'gill' slits, postanal tail, myomeres, and so on. The special vertebrate characteristics include a range of features that make up a true head: well-defined sensory organs (nose, eye, ear) with the necessary nervous connections, the cranial nerves, and the olfactory, optic, and auditory (otic) regions that make up a true brain. Larval sea

Basic Vertebrate
Fig. 1.9 The hypothetical 'basic' vertebrate body plan, shown in longitudinal section. (After Jefferies, 1986.)

squirts and amphioxus have an expansion of the nerve cord at the front end and all the vertebrate cell and sensory organ systems, as we have seen, but these are not developed to the same level as in vertebrates.

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