ACCESSORY DIGESTIVE STRUCTURES

     The digestive system is composed of organs of the gastrointestinal tract through which ingested food passes and a number of accessory digestive structures whose function facilitates digestion although food never actually passes through these structures.  In humans, the liver, pancreas, and gall bladder are accessory structures which release secretions into the small intestines through ducts.  The mouth possesses teeth, a tongue, and salivary glands which are needed for chewing.

THE MOUTH

Although jawless fish possess tongues, the tongues of jawless fish are not homologous to the tongues of higher vertebrates.  The tongues of gnathostome fish are supported by the hyoid bone but lack significant muscles and can achieve only minimal movements.  Fish tongues may possess sensory receptors, papillae, or even teeth. Some amphibians possess a tongue similar to those of fish, some toads have lost their tongues altogether, and other amphibians possess highly modified, mobile tongues.   Perhaps the evolution of more muscular tongues was a result of the pharynx no longer being used in respiration and the need to moisten food in terrestrial environments.    The mammalian tongue is the most highly developed vertebrate tongue which is formed by 5 separate embryonic structures which fuse during development. 

MONKEY

JAW

    Fish possess taste buds which are not restricted to the mouth; they can actually cover the body.  In tetrapods they are located in the mouth and pharynx while in mammals they are concentrated on the tongue (Romer, p. 328).   Human taste buds are depicted in the following two images.
TASTE BUDS
TASTE BUDS

GAR

GAR

GAR
     Although all gnathostomes have lips (Torrey, 1979, p. 317),  movable lips occur only in mammals (except in the platypus and whales) and evolved together with facial muscles (Weichert, 1970, p. 158).  Mammals also developed cheeks and a variety of mammals have developed additional cheek pouches (such as some rodents, some monkeys, and the platypus) (Romer, p. 326).  Modifications of the jaw and jaw musculature allow chewing in mammals which other vertebrates are unable to perform (other than many extinct ornithishian dinosaurs). TONGUE

TEETH

      Although jawless fish and fossil conodonts have tooth-like structures in the mouth, only gnathostomes possess true teeth.

HAGFISH

HAGFISH

CONODONT
CONODONT ELEMENTS

    Most sharks possess a homodont set of teeth (all the teeth have the same shape), but a few sharks possess a heterodont dentition with sharp tearing teeth in front and crushing teeth located posteriorly. (Webster, 1974).  In many fish teeth are attached to bone by collagen fibers while in others they are located in deep sockets (Webster, 1974). A number of dinosaurs (such as the basal ornithopod in the following illustration) evolved a heterodont condition with different kinds of teeth.

HETERODONTOSAURUSteeth

The sarcopterygians which evolved into amphibians possessed teeth with many grooves (giving a labyrinthine appearance in cross section).  The first amphibians were called labyrinthodonts because of their retention of this condition (Webster, 1974, p. 334). 

    While the most primitive teeth were simply located in shallow grooves in the jaws, mammalian teeth are embedded in sockets (Kardong, p. 497).  In mammals, an additional layer of teeth known as cementum anchors the root of the teeth (Weichert, 1970, p. 169).  Monotremes possess horny egg teeth, as in reptiles, to help them escape from their shell when they hatch. (Weichert, 1970, p. 173).  Because mammalian teeth are only replaced once, selection has resulted in occluding surfaces in the upper and lower jaws. 

Mammals have heterodont teeth (different kinds): incisors, canines, premolars, and molars.  The number of teeth has decreased over the course of mammalian and primate evolution.  The catarrhine primates (Old World monkeys, apes, humans) all have the dental formula 2-1-2-3/2-1-2-3 (2 incisors, 1 canine, 2 premolars, and 3 molars); more primitive primates have an additional premolar.

     .  The only digestive glands in the mouths of fish are simple mucuous glands.  Once vertebrates adapted to terrestrial environments, glands became necessary to moisten food before swallowing.  A number of oral glands are known from modern amphibians (including the lingual glands) and from modern reptiles (including the lingual and sublingual glands).  Mammals develop parotid, submandibular (or submaxillary), and sublingual glands.  There have been a few modifications in the ancestral set of salivary glands in mammals: rabbits and horses lack the submandibular glands while mice and some insectivores lack the sublingual glands.  The salivary glands in general are reduced or absent in whales and sirens. Artiodactyls possess additional molar glands and dogs possess orbital glands (Weichert, 1970, p. 161-2).

PANCREAS

 Amphioxus lacks a pancreas.  In jawless fish, diffuse patches of pancreatic tissue exist which empty separately into the small intestine, similar to the multiple units of the embryonic pancreas of higher vertebrates (Romer, p. 393-5).    In jawed vertebrates , the units of the pancreas unite during embryological development and the exocrine and endocrine cells fuse to form a composite structure (Kardong, p. 519). 

PERCH

PERCH

FROG

FROG

ALLIGATOR

ALLIGATOR

OPOSSUM

OPOSSUM

CAT

CAT

HUMAN MODEL

MODEL