48 COI^IPARATrVE ANATOMY. 



by the anus. This differentiation of the digestive tube into 

 several unequal parts is the most important complication which 

 it undergoes ; any further differentiations are subordinate to this. 

 Three tracts are accordingly henceforward distinguished, as fore- 

 gut, mid-gutj and hind-gut. 



In addition to the varying and numerous changes in size which 

 the different portions of the canal undergo, other arrangements, due 

 either to special new functions, or mere expressions of further 

 division of labour, arise in it. Organs for seizing and comminuting 

 the food become attached to the mouth, or mark off a portion of the 

 oesophagus (masticatory organs). In the stomach also there 

 are sometimes masticatory organs of this kind. When they occur at 

 the commencement of the oesophagus, just behind the mouth, this 

 part, which is frequently distinguished by its larger supply of 

 muscles, is known as the pharynx. 



The size of the cavity of the canal is increased by dilatations, or 

 caecal diverticula. Crops are formed in the course of the oesophagus, 

 caecal sacs on the stomach and on the rest of the intestine, which 

 are variously complicated in number and arrangement. When the 

 length of the alimentary canal is greater than that of the body, it 

 is arranged in ascending and descending loops, or in coils, and so 

 adapted to the size of the cavity in which it is contained. Both the 

 quantity and quality of the food ingested is of the greatest import- 

 ance as affecting all these relations of parts ; and nowhere is the 

 adaptation of the organ to its function — which results from the 

 mode of life of the animal — more clearly seen than in the arrange- 

 ments of the alimentary canal. 



Secretory organs are generally connected with the alimentary 

 canal, to aid in the process of digestion ; their products dissolve, 

 and act on the nutrient matter by chemical change. Glands of 

 this kind are sometimes distributed over the whole canal, and some- 

 times distinguish certain portions of it only. In their simplest form 

 they are not differentiated from the enteric wall, and in that case 

 are not distinctly marked-off parts. Those marked off from the wall 

 of the enteron are separated into two chief divisions. One of them 

 comprises the glands which open in the buccal cavity, or its neigh- 

 bourhood, and are distinguished as salivary glands. Another 

 group is formed in the portion which serves for digestion, and is 

 regarded as a bile-producing organ, a liver. It is right to note that 

 the distinguishing of these organs by names which are applied to 

 organs of higher organisms, physiologically better understood, is 

 merely hypothetical, for nothing is known of the physiology of 

 most of the organs of the lower animals. This holds especially for 

 the epithelium of the gut, which generally appears coloured, and 

 which is often called the " liver. ^^ This organ appears under the form 

 of an epithelium, lining a part of the digestive cavity in the Coelen- 

 terata, in many Vermes, and even in Insects, till at last it becomes 

 limited to definite cascal appendages of the alimentary canal, and so 

 attains to the lowest grade of independence. The liver presents 



