Digestive system includes alimentary canal and the associated glands. Alimentary canal or gut shows four distinct regions, namely, ingressive zone that includes mouth, lips and jaws for capturing and handling food; progressive zone includes oral cavity, pharynx and oesophagus through which food passes with little digestion; degressive zone contains stomach and small intestine in which digestive process is accomplished and the egressive zone includes colon and rectum where undigested food stays before expulsion and excess water from it is absorbed. Each part of the alimentary canal is described below one by one in a sequence.
MOUTH
Mouth is the anterior opening of the alimentary canal, which may be terminal, ventral or slightly dorsally directed. It is guarded by suckers in cyclostomes and by jaws and teeth in gnathostomes. Lips are horny in fishes but fleshy and suctorial in mammals. Birds and turtles possess horny beak in place of lips.
ORAL CAVITY
This is the anterior most chamber of alimentary canal meant for handling food. Depending upon the kind and size of food, its size is highly variable and it contains three important organs for handling food material, namely, oral glands, tongue and teeth.
ORAL GLANDS
Cyclostomes possess mucous glands in oral cavity but Petromyzon also has salivary glands that secrete an anticoagulant enzyme. Fishes and perennibranch amphibians have no particular oral glands except simple mucous glands. In tailed amphibians and apoda oral glands are almost nonexistent but in anurans there are. Oral glands are poorly developed in turtles and crocodiles but well developed in lizards and snakes. Poison glands of snakes are modified labial glands and those of Heloderma, the only poisonous lizard that has fangs in the lower jaw, has sublingual glands modified as poison glands.
Birds possess sublingual glands that open into the floor of the oral cavity. Mammalian oral cavity is very wet as it contains two types of glands – salivary glands and mucous glands. The compound acinar submaxillary or submandibular glands lie in the posterior part of the lower jaw. Sublingual glands are smaller than the other two salivary glands. Molar glands are mucous glands that are well developed in herbivores and open near the upper molars. Another kind of mucous glands are Orbital glands which occur in cat and dog family.
TONGUE
Tongue is a fleshy and highly mobile organ in the oral cavity that is used in various ways in vertebrate groups.
Cyclostomes possess a thick and fleshy primary tongue on the floor of the oral cavity. In fishes the tongue is primary and merely a fleshy fold on the floor of anterior end of pharynx supported by the extension of hyoid arch.
In perennibranch urodeles such as Necturus, tongue is similar to fishes and is not put to much use. Frogs and toads having a predilection for insectivorous diet are gifted with a highly flexible tongue that consists of a basal primary tongue and the anterior glandular and muscular secondary tongue. Turtles and crocodiles being amphibious in nature have a small non-protrusible tongue but snakes and lizards possess a highly movable tongue that is bifurcated at the apex and supplied with olfactory cells.
Bird tongue is short and hard and practically lacks muscles and lateral lingual swellings. Such incapable tongue is of no handicap to these beaked creatures as the food does not stay in the mouth for longer duration. However, some birds are gifted with long and flexible tongue such as woodpeckers.
Mammalian tongue is the best developed of all vertebrates, except in the aquatic cetaceans. It is derived from 5 portions—paired fleshy ridges of hyoid arch, a median secondary tongue called tuberculum impar and paired lateral lingual swellings, which provide it extraordinary mobility and flexibility in the oral cavity. Tongue of anteaters is suitably long and sticky to feed on termites which they dig out by their strong fossorial front legs.
TEETH
Teeth are hard bony structures in the oral cavity that are variously modified to capture, tear, cut or grind food material before it is swallowed. Epidermal teeth are hard cornified epidermal structures of rare occurrence, as in the buccal funnel of cyclostomes and on the edges of tadpole jaws.
TYPES OF TEETH
Polyphyodont dentition involves replacement of teeth from time to time several times in lifetime so that jaws are never left without teeth.
Diphyodont dentition is a characteristic of mammals in which milk teeth appear in the young ones but as they grow and jaw becomes larger, milk teeth are replaced by larger permanent ones.
Monophyodont teeth appear only once in lifetime and if they fall they are never again replaced by new ones.
Based on the type of attachment of teeth on the jaw bone the following three types are found in vertebrates:
Acrodont teeth are attached on the top surface of the jaw bone as in fish and amphibians.
Pleurodont teeth are attached on the inner side of the jawbone that brings larger surface area of tooth in contact with jawbone and hence attachment is stronger, as in lizards and urodeles.
Thecodont dentition is found in mammals in which root of the tooth is firmly fixed in a socket of the jawbone, making the attachment strongest in vertebrates.
Based on the kinds of teeth found there are two types of dentition:
Homodont dentition is found in the majority of vertebrates such as fish, amphibia and reptiles in which all teeth are functionally of the same type, although their size may be variable depending on the location.
Heterodont dentition occurs in mammals in which there are 4 functionally different types of teeth, namely, flat incisors for cutting, long and pointed canines for tearing flesh and large and broad premolars and molars with flat grinding surface.
There are also some other type of teeth as follows:
Secodont teeth have sharp cutting edges that function like scissors to cut flesh.
Bunodont teeth are small with smaller cusps or tubercles on the surface for handling soft diet as in man, monkeys, rodents etc.
Brachydont teeth are smaller and low crowned suitable for feeding on soft diet.
Hypsodont teeth possess larger crown that can resist wear and tear of feeding on tough and fibrous diet as in ungulates.
Selenodont teeth, as found in horses and other ungulates, deposit silica around cusps and in the depressions of the grinding surface.
Lophodont teeth are found in elephants which feed on the roughest diet that any mammal can feed on.
DEVELOPMENT OF TOOTH
Teeth develop over the jaw bone where certain malpighian cells start actively multiplying forming a mass of cells called dental lamina or enamel organ. A dental papilla made of group of dermal cells appears below the dental lamina that supplies nourishment to the growing mass of cells. Cells of the outer layer of dermal papilla arrange themselves in a row and get differentiated into odontoblast cells.
Epidermal cells of the dental lamina that cover the growing dentine are called ameloblasts. The tooth gradually grows outwards and eventually gets exposed by penetrating through the skin covering the jaw bone. The dental papilla inside the pulp cavity remains active along with its blood supply and nerve intact. This development of tooth is identical to the development of dermal scales in fishes. Hence shark teeth are also called modified placoid scales.
COMPARATIVE ACCOUNT OF DENTITION
Cyclostomes have only epidermal teeth. Some fishes are toothless such as sturgeons, sea horse and pipe fish and others like lung fishes and Chimaera have teeth modified into crushing plates. Majority of fishes possess Polyphyodont, Acrodont and homodont dentition suitable for seizing prey.
In amphibians teeth are located on jaw bones, palatine and vomer bones and are Polyphyodont. Tadpoles lack true teeth and their jaws have horny epidermal ridges which are used to scrape algae on which they feed.
Among reptiles, turtles lack teeth and have horny beak. In others, teeth are generally confined to jaw bones but in snakes and lizards may occur on palatine and pterygoid bones. Fangs of snakes are modified upper maxillary teeth.
Modern birds lack teeth but Archaeopteryx had thecodont dentition and so were the toothed birds Ichthyornis and Hesperornis.