SIMPLEST FORMS OF DIGESTIVE APPARATUS. 253 



chemical precipitation ; and a cavity or sac in which these operations 

 may be performed. 



443. In the lowest Animals, we find this cavity formed upon a very 

 simple plan ; the digestive sac being a mere excavation in the solid tissue 

 of the body, lined with a membrane which is an inverted continuation of 

 the external integument, and communicating with the exterior by one 

 orifice only, through which food is drawn in, and excrementitious matter 

 rejected. In the little Hydra, or fresh-water Polype, the external 

 covering of the body and the lining of the stomach correspond so closely 

 in their structure, their actions differing only with their situation, as to 

 be mutually convertible ; for the animal may be turned completely in- 

 side-out, without its functions being deranged: The fluid necessary to 

 dissolve the food, known by the name of "gastric fluid" or "gastric 

 juice" is secreted in the walls of the stomach ; and, from the trans- 

 parency of the tissues, the whole process may be watched. The prey 

 is frequently, and indeed generally, introduced alive, by the contractile 

 power of the arms, which coil round it, and gradually draw it into the 

 mouth or entrance to the stomach ; and its movements may often be 

 observed to continue for some time after it has been swallowed. In a 

 little time, however, its outline appears less distinct, and a turbid film 

 partly conceals it ; the soft parts are soon dissolved and reduced to a 

 fluid state ; and any firm indigestible portions which the body may con- 

 tain, are rejected through the aperture by which it entered. The nutri- 

 tive matter is absorbed by the walls of the stomach, every part of which 

 appears to be endowed with equal power in this respect : ami it is con- 

 veyed to the remoter parts of the arms by the simple imbibition of one 

 part from another, without any proper circulation through vessels. 



444. In Polypes of a higher conformation, however, the digestive 

 cavity is provided with a second orifice : from the dilated cavity or 

 stomach, an intestinal tube proceeds ; and this has a termination dis- 

 tinct from the mouth, though often in its neighbourhood. The food, 

 before entering the stomach, is submitted to a powerful triturating ap- 

 paratus, resembling the gizzard of birds, by which it is broken down ; 

 and in the digestive cavity it is submitted, not merely to the action of 

 the gastric fluid, but also to that of the bile, which is secreted in little 

 follicles in the walls of the stomach, arid which is poured into its cavity 

 during the process of digestion, being easily recognised by its bright 

 yellow colour. The excrementitious matter is rejected in the form of 

 little pellets, through the intestinal tube. . 



445. As we ascend the Animal scale, we find the digestive apparatus 

 gradually increased in complexity ; but its essential characters remain 

 the same. Near the entrance to the stomach, we usually find an appa- 

 ratus for effecting the mechanical reduction of the food, by which its 

 subsequent solution may be rendered more easy. This may consist of 

 a set of teeth ; either fixed in the mouth, as in Mammalia and Reptiles ; 

 or more particularly besetting the pharynx, as in Fishes ; or attached 

 to the walls of the stomach, as in Crustacea. Or it may be formed by 

 the tongue, converted into a sort of rasp ; as in the ^common Limpet, 

 which thus reduces the sea-weeds that constitute its chief food. ^ Or t 

 same purpose may be answered by a gizzard, or first stomach, with dense 



