Book - Vertebrate Zoology (1928) 26
Vertebrate Zoology G. R. De Beer (1928)
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Chapter XXVI The Alimentary System
The alimentary system comprises the tube which leads from mouth to anus, together with the glands attached to it which aid in the processes of digestion. There is a slight invagination of the ectoderm at the mouth and anus, forming the stomodaeum and the proctodeum ; but the remainder, which forms by far the larger part of the alimentary system, is formed exclusively from the endoderm. In addition to the digestive glands, the alimentary canal has a number of derivatives which have been considered in connexion with other organ-systems. So the gill-pouches and the larynx and lungs belong to the respiratory system ; the allantoic bladder forms part of the excretory system ; while the thyroid gland, which in Gnathostomes and adult Cyclostomes is one of the endocrine organs, belongs to the alimentary system in Amphioxus and the larval Cyclostome (Ammocoete).
The primitive method of obtaining food is by the creation of a current of water towards the mouth by means of cilia. This is the case in Amphioxus (and the Ascidians). Here the endostyle is accessory to the alimentary system in that it ensures that the particles of food reach the intestine instead of being lost with the current of water flowing out through the gill-slits. The method of feeding by means of a sucking mouth and a rasping tongue which is characteristic of the Cyclostomes, is secondary and specialised. In all the Gnatho- stomes, the most anterior visceral arches, between the mouth and the first visceral cleft, become modified and adapted for seizing food, and give rise to the jaws. This method enables food of larger size to be obtained than is possible by the ciliary method, and the Gnathostomes were thereby able to evolve to greater size. In these forms also, the jaws are garnished with teeth, and the nature and shape of the teeth varies with the kind of diet. Not only do teeth assist in seizing prey, but in the higher forms they serve to grind it up small, which is an aid to the processes of digestion. In the higher verte- brates, the tongue may also be used for obtaining food as in the case of the chamaeleon, and it assists in the process of swallowing.
In the primitive forms the alimentary canal or gut runs straight from mouth to anus, as in Amphioxus and the Cyclo- stomes. In these two forms the lining of the gut is ciliated, but in higher forms the ciliation is restricted to certain anterior regions. Food is propelled along by peristaltic action of the smooth muscle in the gut- wall.
Beginning in the Selachians, a special part of the gut is modified as a receptacle in which food is treated with digestive juices secreted by its walls, and in which absorption does not take place. This is the stomach, and in all Gnathostomes it is an enlarged region of the gut, kinked to the left side of the body, and situated between the non-digestive supply-tube or oesophagus and the absorbent intestine. The intestine of the Gnathostomes is greater in length than the space which con- tains it, with the result that it is more or less coiled. In the higher forms the intestine is very considerably longer than the body itself. The effect of this is to increase the surface of absorption. A modification which serves the same function is the spiral valve in the intestine, which is feebly developed in Petromyzon and well developed in the Selachians. The spiral valve is also present in Ceratodus and in a few primitive bony fish (Teleostomes), but it is lost in all higher forms. A peculiarity of the stomach of the higher bony fish (Teleostei) is the development of a number of blind outgrowths (pyloric coeca) from the hinder end of the stomach. The wall of the intestine is well supplied with blood-vessels belonging to the hepatic portal system, and with lymphatic vessels or " lacteals."
The oesophagus in birds is modified and enlarged into a crop or temporary storage place. The stomach is divided into two regions. The first of these, the proventri cuius, has soft walls provided with glands. Next comes a hard- walled gizzard, in which the food is crushed with the help of stones, for the bird has no teeth and so cannot perform this function in the mouth.
In mammals, the stomach is simple except in a group of the Ungulates called the Ruminants, where it is divided into several parts. These animals " chew the cud," and their stomach is modified in consequence. The food (grass) is swallowed down (without being masticated) into the anterior divisions of the stomach composed of the paunch and the " honey-comb." When the animal ceases feeding, the food is brought up to the mouth again and thoroughly chewed and salivated. It then redescends to the other divisions of the stomach, termed the maniplies and the abomasum. The latter has glandular walls, and secretes digestive juice.
The region between the intestine and the anus is short and straight in the lower forms, and is called the rectum. It is usually marked off from the intestine by the development of a constriction, the ileo-colic sphincter, and by one or two blind diverticula or coeca. In the Tetrapods the region between the intestine and the anus becomes longer and coiled, and it becomes possible to distinguish a so-called large intestine (on account of its diameter) or colon which is coiled, from the terminal straight rectum. The intestine proper is then called the small intestine. The large intestine is concerned with the absorp- tion of water from the non- digested remains of the food, a function of importance for animals which inhabit dry land.
The ccecum in mammals may be very large, as in the rabbit, and this condition is common in herbivorous animals. The ccecum contains a colony of bacteria whose function it is to attack the cellulose of the food and to digest it. In other forms the ccecum is reduced, and may be represented only by its tip, the vermiform appendix, as in man.
The anus primitively opens to the outside in conjunction with the urino-genital ducts, forming a cloaca. This con- dition is departed from in the higher bony fish (Teleostomes) and in the higher mammals or Ditremata (Marsupials and Placentals), in which the alimentary and urino-genital systems open separately to the exterior.
The first special digestive gland to appear is the liver, which is present in Amphioxus. In the higher forms it connects with the anterior part of the intestine by the bile- duct, and usually possesses a gall-bladder. A rudimentary pancreas appears in the Cyclostomes, and in higher forms it is well developed, connecting with the intestine by one or more pancreatic ducts. It may be mentioned that in mammals at least, the secretion of pancreatic juice is started by a substance " secretin," which is released from the lining of the intestine into the blood-stream and carried therein to the pancreas which it stimulates. This is of interest, for secretin was the first of the hormones (chemical stimulants with specific effects and which are carried about in the blood) to be properly recognised.
Salivary glands are lacking from the lower water-living chordata, as is readily understood when it is remembered that a current of water is constantly sweeping through the mouth to the gill-slits. Salivary glands make their appearance in the Amphibia. In the snakes, some of the salivary glands may be modified into poison-glands.
Literature
Oppel, A. Der Magen, Schlund und Darm, Mundhohle, Bauchspeicheldriise und Leber. Lehrbuch der Vergleichenden mikroskopischen Anatomie der Wirbeltiere, Parts I, 2, and 3. Fischer, Jena, 1896.
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- Vertebrate Zoology 1928: PART I 1. The Vertebrate Type as contrasted with the Invertebrate | 2. Amphioxus, a primitive Chordate | 3. Petromyzon, a Chordate with a skull, heart, and kidney | 4. Scyllium, a Chordate with jaws, stomach, and fins | 5. Gadus, a Chordate with bone | 6. Ceratodus, a Chordate with a lung | 7. Triton, a Chordate with 5-toed limbs | 8. Lacerta, a Chordate living entirely on land | 9. Columba, a Chordate with wings | 10. Lepus, a warm-blooded, viviparous Chordate PART II 11. The development of Amphioxus | 12. The development of Rana (the Frog) | 13. The development of Gallus (the Chick) | 14. The development of Lepus (the Rabbit) PART III 15. The Blastopore | 16. The Embryonic Membranes | 17. The Skin and its derivatives | 18. The Teeth | 19. The Coelom and Mesoderm | 20. The Skull | 21. The Vertebral Column, Ribs, and Sternum | 22. Fins and Limbs | 23. The Tail | 24. The Vascular System | 25. The Respiratory system | 26. The Alimentary system | 27. The Excretory and Reproductive systems | 28. The Head and Neck | 29. The functional divisions of the Nervous system | 30. The Brain and comparative Behaviour | 31. The Autonomic Nervous system | 32. The Sense-organs | 33. The Ductless glands | 34. Regulatory mechanisms | 35. Blood-relationships among the Chordates PART IV 36. The bearing of Physical and Climatic factors on Chordates | 37. The origin of Chordates, and their radiation as aquatic animals | 38. The evolution of the Amphibia : the first land-Chordates | 39. The evolution of the Reptiles | 40. The evolution of the Birds | 41. The evolution of the Mammalia | 42. The evolution of the Primates and Man | 43. Conclusions | Figures | Historic Embryology
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