El Juego Cuando se prepara un partido

Th e basic endoskeletal structural features of chordates are the notochord, acting as a dorsal stiff ening rod running along the length of the body, and some sort of gill skeleton that keeps the gill slits open. Th e cranium surrounding the brain was the fi rst part of the verte-brate skeleton to evolve. Next the dermal skeleton of external plates and the axial skeleton (vertebrae, ribs, and median fi n supports) were added, and still later the appendicular skeleton (bones of the limb skeleton and limb girdles) evolved.

The Cranial Skeleton Th e skull, or cranium, is formed by three basic components: the chondrocranium (Greek chondr = cartilage [literally “gristle”] and cran = skull) surrounding the brain; the splanchnocranium (Greek splanchn = viscera) forming the gill supports; and the dermatocranium (Greek derm = skin) forming in the skin as an outer cover that was not present in the earli-est vertebrates.

Th e splanchnocranial components of the vertebrate skeleton are known by a confusing variety of names.

In general they can be called gill arches because they support the gill tissue and muscles. Th e anterior ele-ments of the splanchnocranium are specialized into nongill-bearing structures in all extant vertebrates, such as the jaws of gnathostomes. Other names for these structures are pharyngeal arches (because they form in the pharynx region) and branchial arches (which is just a fancy way of saying gill arches be-cause the Greek word branchi means gill). Yet another name for these structures is visceral arches , because the splanchnocranium is also known as the visceral skeleton. (Still another name associated with the pharyngeal region , aortic arches, refers not to the gill skeleton but to the segmental arteries that supply the gill arches.)

means “shell-skinned”), forming a type of exoskel-eton. We think of vertebrates as possessing only an endoskeleton, but most of our skull bones are dermal bones, and they form a shell around our brains. Th e en-doskeletal structure of vertebrates initially consisted of only the braincase and was originally formed from cartilage. Th us, the condition in many early verte-brates was a bony exoskeleton and a cartilaginous en-doskeleton ( Figure 2–7 ).

Teeth

Teeth form from a type of structure called a dermal pa-pilla, so they form only in the skin, usually over der-mal bones. When the tooth is fully formed, it erupts through the gum line. Replacement teeth may start to develop to one side of the main tooth even before its eruption. Th e basic structure of the teeth of jawed ver-tebrates is like the structure of odontodes , which were the original toothlike components of the original

Enamel Dentine

Pulp cavity

© 1998 The McGraw-Hill Companies, Inc.

Cementum Periodontal ligament

Enamel organ Dermal papilla

Replacement tooth Spongy acellular bone

Basal acellular bone Dentine tubercles

(b)

(a)

Figure 2–6 Organization of vertebrate mineralized tissues.

(a) Th ree-dimensional block diagram of dermal bone from an extinct jawless vertebrate (heterostracan ostracoderm).

(b) Section through a developing tooth (shark scales are similar).

of the structure and early evolution of the vertebrate cra-nium, and Figure 2–9 (see page 36) illustrates three vertebrate crania in more detail.

The Cranial Muscles Th ere are two main types of stri-ated muscles in the head of vertebrates: the extrinsic eye muscles and the branchiomeric muscles. Six mus-cles in each eye rotate the eyeball in all vertebrates ex-cept hagfi shes, in which their absence may represent secondary loss. Like the striated muscles of the body, these muscles are innervated by somatic motor nerves.

Th e branchiomeric muscles are associated with the splanchnocranium and are used to suck water into the mouth during feeding and respiration. Branchiomeric muscles are innervated by cranial nerves that exit from the dorsal part of the spinal cord (unlike striated muscles, which are innervated by motor nerves that exit from the ventral part of the spinal cord). Th e rea-son for this diff erence is not clear, but it emphasizes the extent to which the vertebrate head diff ers in its structure and development from the rest of the body.

The Axial Skeleton and Musculature Th e notochord is the original “backbone” of all chordates, although it is never actually made of bone. Th e notochord has a core We will call these structures pharyngeal arches when

we are discussing the embryonic elements of their de-velopment, and gill arches in adults, especially for those arches that actually do bear gill tissue (i.e., arches 3–7).

Th e vertebrate chondrocranium and splanchnocra-nium are formed primarily from neural-crest tissue, although a splanchnocranium-equivalent formed by endodermal tissue is present in cephalochordates and hemichordates. Th us, a structure with the same func-tion as the vertebrate splanchnocranium preceded the origin of vertebrates and of neural-crest tissue, although only vertebrates have a true splanchnocranium (i.e., one that is derived from neural-crest tissue).

Th e chondrocranium and splanchnocranium are formed from cartilage in the ancestral vertebrate con-dition, but they are made of endochondral bone in the adults of some bony fi shes and most tetrapods. Th e der-matocranium is made from dermal bone, which is formed in a membrane rather than in a cartilaginous precursor.

(Because it forms in a membrane it is sometimes called membrane bone.) Th e dermatocranium is cartilaginous only as a secondary condition in some fi shes, such as sturgeons, where ossifi cation of the dermatocranium has been lost. Figure 2–8 shows a diagrammatic representation

The dermal skeleton or exoskeleton

(a)

(b)

The endodermal skeleton or endoskeleton Axial

Cranial

Appendicular

Figure 2–7 Vertebrate skeletons.

(a) the originally dermal bone exoskeleton and (b) the originally cartilaginous bone endoskeleton. (Th e animal depicted is an extinct bony fi sh.)

(a) Basic vertebrate cranium

Ventral view of basic primitive vertebrate (b) Lamprey

(c) Ostracoderm (d) Chondrichthyan (e) Osteichthyan

Lateral Views Cross-sectional Views

(at level of dotted line in lateral views)

Chondrocranium (3-7 plus 3 additional) form complex form oral cartilages in this region

Chondrocranium

Chondrocranium Gill slit that was here

was squeezed out and

Gill arches are now more complexly hinged

tion, a feature that possibly fi rst appeared as a startle response in larvae. Th e notochord stiff ens the body so it bends from side to side as the muscles contract.

(Without the notochord, contraction of these muscles would merely compress the body like an accordion.)

Most fi shes still use this basic type of locomotion. Th e paired fi ns of jawed fi shes are generally used for steering, braking, and providing lift–not for propulsion except in some specialized fi shes such as skates and rays that have winglike pectoral fi ns and in some derived bony fi shes (teleosts) such as seahorses and coral reef fi shes.

Energy Acquisition and Support