30 Introductio7i to A nimal Morphology. 



protoplasm of the plastide or cytocorm, or by means 

 of a funnel and wide passage, which sometimes 

 narrows into a tube (CEsophagus). Polystomata, &c., 

 have an internal cavity added, and in Coelenterates 

 we find this lined by separate cells,* which secrete 

 a special fluid by which the food is dissolvedf, and 

 the nutritive part transuding the plastides of the 

 walls nourishes the animal. Effete matters are ejected 

 either at the mouth or by a special opening at the 

 distal end (anus). An elongation of the communi- 

 cation between the stomach and anus to increase the 

 absorbing surface forms an intestine from which, for 

 the same purposes, blind pouches (caeca) may project. 

 Secondary appendages for the prehension and me- 

 chanical division of the food (jaws and teeth), for the 

 moistening of the food in deglutition (salivary glands J), 

 and for the assimilation of fats (liver and pancreas§), 

 are afterwards developed in and from the tube. 



* The development of an internal cavity is regarded by Haeckel as dis- 

 tinguishing the second or gastraea type of structure from the primitive 

 planula form. 



t Gastric juice contains in higher animals water, pepsin (an albuminoid), 

 hydrochloric, butyric, and lactic acids, and phosphates of soda and lime, 

 &c. It converts albuminoids into dialysable forms (peptone, metapeptone, 

 parapeptone, &c.) by making them take up water. 



X Saliva, the secretion of the parotid, contains water, ptyalin (an albumi- 

 noid), albumin, mucin, sulpho-cyanide of potassium, a fatty acid, and salts of 

 potass, soda, hme, &c. This fluid can saccharize starch in neutral alkaline or 

 weakly acid solutions. The secretion is mainly poured out under the agency 

 of stimuli, food, odours, &c. Other glands, submaxillary, sublingual, buccal, 

 labial, &c., are mucous glands pouring fluid into the mouth. The 

 secretion of the first of these acts feebly in saccharizing starch. These 

 secretions are but shghtly influenced by the smell of food, but are actively 

 affected by the movements of mastication. 



§ The liver secretes bile, an orange or yellow alkaline bitter fluid con- 

 taining water, cholesterin (a crj'stalline fat CsiH^iO,), sodic glychochoJate, 



