Spectrum Biology - September 2016

Complete Digestive Tract :

An Evolutionary Breakthrough

The development of the anus and complete digestive tract in the

aschelminths was an evolutionary breakthrough. A complete digestive

tract permits the oneway flow of ingested food without mixing it with

previously ingested food or waste. Complete digestive tracts also have

the advantage of progressive digestive processing in specialised

regions along the system. Food can be digested efficiently in a series

of distinctly different steps.

Bivalve Molluscs

 Many bivalve molluscs are suspension feeders and ingest

small food particles. The digestive tract has a short

oesophagus opening into a stomach, midgut, hindgut

and rectum.

 The stomach contains a crystalline style, gastric

shield and diverticulated region.

 The midgut, hindgut and rectum function in

extracellular digestion and absorption.

 Digestion is a coordination of three cycles, i.e., feeding,

extracellular digestion and intracellular digestion.

 The mechanical and enzymatic breakdown of food

during feeding provides the small particles for

intracellular digestion.

 Intracellular digestion releases the nutrients into the

blood and produces fragmentation spherules that

both excrete wastes and lower the pH for optimal

extracellular digestion.

 These three cycles are linked to tidal immersion and

emersion of the molluscs.

Diversity in Digestive Structure

of Vertebrates

The complete vertebrate digestive tract is highly specialised

in both structure and function for the digestion of a wide

variety of foods. The major digestive structures discussed

below illustrate the diversity of form and function among

different vertebrates.

Tongue

A tongue or tongue-like structure develops in the floor of the

oral cavity in many vertebrates.

Diversity in the Tongue of Vertebrates

Or gan isms Tongue Fig ure

Lamprey

Lamprey has a protrusible

tongue with horny teeth

that rasps its prey’s flesh.

Fish

Fishes may have a

primary tongue that bears

teeth that help in holding

the prey. This type of

tongue is not muscular.

Frog,

salamanders

and some

lizards

They can rapidly project

part of their tongue from

the mouth to capture an

insect.

Woodpecker

It has a long, spiny tongue

for gathering insects and

grubs.

Cat and

other

carnivores

Spiny papillae on their

tongues help them rasp

flesh from a bone.

Teeth

 Most of the vertebrates have teeth except birds, turtles

and baleen whales.

 Birds lack teeth probably to reduce body weight for flight.

 Teeth are specialised, depending on whether an animal

feeds on plants or animals and on how it obtains its food.

 The teeth of snakes slope backward to aid in the

retention of prey while swallowing.

No wonder you can feel your heartbeat so

easily. Pumping blood through your body

quickly and efficiently takes quite a bit of

pressure resulting in the strong contractions of the heart and the

thick walls of the ventricles which push blood to the body. The

human heart creates enough pressure to squirt blood 30 feet.

Digestive

diverticula

Left

pouch

Right pouch

Major

typhlosole

Midgut

Style sac

Crystalline style

Gastric shield

Oesophagus

Fragmentation

spherules

(a) Food (b)

Large food

particles

(c)

(d)

(e)

Digestion in bivalve molluscs

Tongue

Tongue

Tongue

Tongue

Spiny papillae on tongue