28 



DIGESTIVE CANAL. 



which are superadded to these as we ascend in 

 the scale either form an extension of the nutri- 

 tive apparatus, or are destined to regulate the 

 kind of food admitted into the alimentary 

 cavity. An animal, in the abstract, may almost 

 be viewed as a moving sac, organized to con- 

 vert foreign matter into its own likeness, and 

 all the complex organs of animal life are but 

 auxiliaries to this primitive digestive bag. The 

 bones and other hard parts which form the 

 solid frame-work of the body connected toge- 

 ther by their various ligaments serve only as 

 firm levers to enable the active organs, the 

 muscles, to carry it to and fro, and the ner- 

 vous system with its various organs of sense 

 serve but to direct its motions in quest of food. 

 Nature has placed the unorganized food of 

 plants on the exterior of their body, and their 

 vessels are sent there to seek it, which roots 

 them through life to a fixed point; but animals 

 place their food in their stomach and have their 

 roots directed inwards and towards that central 

 reservoir, so that they can move about and 

 select what is most congenial to their nature. 

 The organs of animal life relate to this diffe- 

 rence between the two organized kingdoms 

 to this locomotion of animals and their power 

 of selecting their food; but the organs of vege- 

 tative life of which the alimentary canal is the 

 first, relate merely to the assimilation of food 

 when already within the body, and are there- 

 fore common to animals with plants. The 

 digestive surface of the plant is the surface of 

 its root, ramified and fixed in the soil, which 

 affords it a never-failing supply of food ; so 

 that the vegetable is like an animal with its 

 stomach turned inside out. The organs of 

 relation are necessarily connected with the 

 varied circumstances in which animals are 

 placed, and are remarkable for their variable 

 character, and even for their inconstancy in the 

 lower tribes, where they are often entirely want- 

 ing ; but those of vegetative or organic life are 

 more regular and constant in their character, 

 and indeed no organ is more universal among 

 animals than that internal digestive cavity by 

 which they differ so much from the species of 

 the vegetable kingdom. This primitive sac is 

 but a development or a continuation of the 

 mucous surface of the skin, which extends 

 into the homogeneous cellular tissue of the 

 body, or completely through it; and although, 

 in the simplest conditions of animals, it per- 

 forms alone all the assimilative functions, we 

 find it, as we ascend in the scale, giving origin 

 to various other systems to which distinct parts 

 of the complex function of assimilation are 

 entrusted. Thus the peripheral mode of nutri- 

 tion of the plant passes insensibly into the 

 central internal mode of the animal, and all 

 the organs of organic life, whether they open 

 into the digestive cavity within, or on the 

 surface of the body without, may be considered 

 as originating from the skin, which is itself only 

 a portion of the primitive cellular tissue of the 

 body, here modified by the contact of the sur- 

 rounding element so as to assume the character 

 of a mucous membrane. As the various tubular 

 prolongations become more and more developed 



and isolated from this primitive source, they 

 assume properties more and more peculiar, 

 and thus form the numerous glandular appara- 

 tus and vascular systems. 



An internal digestive cavity, the first element 

 of all the organs subservient to individual 

 nutrition, is observed in every class of animals 

 and almost in every genus ; and where this part 

 has not yet been perceived, there can be little 

 doubt, from analogy, of its existence. Its 

 form and structure vary according to the kind 

 of food on which the various tribes of animals 

 are destined to subsist, and the extent of elabo- 

 ration it requires to undergo to assimilate it to 

 the animal's body; so that the diversities of 

 this first part of the digestive apparatus are 

 intimately related to all the living habits of 

 animals, and to all the peculiarities they pre- 

 sent in their other assimilative organs and in 

 their organs of relation. 



1. Polygastrica. In the monads a digestive 

 apparatus is distinctly seen, and in almost all 

 the other genera of animalcules, where, indeed, 

 the internal cavities connected with this im- 

 portant function are so numerous in almost all 

 the known forms of these animals that this 

 lowest class of animals has been termed poly- 

 eastrica to express their common character. 

 From the transparency of these minute animals, 

 their digestive sacs appear, when empty or 

 when filled with water, like portions of the 

 common cellular substance of the body, or 

 like animalcules which have been swallowed, 

 or like internal gemmules; and from not being 

 generally recognized as alimentary cavities, 

 many observers were led to suppose that the 

 animalcules are nourished solely by superficial 

 absorption like marine plants. Leuwenhoeck, 

 however, not doubting that they possessed a sto- 

 mach, believed that they devour each other ; this 

 wasobserved also by Ellis, and Spallanzani main- 

 tained that they devour each other so voraciously 

 that they are seen to become distended with 

 this food. Goeze saw the trichoda seizing and 

 swallowing the animalcules which were smaller 

 than itself. Baron Gleichen, in order to dis- 

 cover the form of their internal digestive cavi- 

 ties, placed them in infusions coloured with 

 carmine which they soon swallowed, and in 

 his coloured plates he has represented this red 

 colouring matter as filling the internal stomachs 

 of numerous trlchodce, vorticellte, and other 

 animalcules. Indeed those internal globular 

 cavities of animalcules are represented in the 

 plates of Miiller, Bruguiere, and all the older 

 writers on this class. But Ehrenberg, by 

 adopting the plan of Gleichen and Tretnbley 

 of employing opaque colouring matter to detect 

 the forms of these internal cavities, and by 

 using principally carmine, sap-green, and indigo, 

 carefully freed from all impurities which might 

 prevent their being swallowed, has succeeded 

 better than all his predecessors in unfolding 

 the structure of the digestive organs of animal- 

 cules. Such coloured organic matter diffused 

 as fine particles mechanically suspended in the 

 water in which animalcules are placed, is 

 readily swallowed by them, and renders visible 

 through their transparent bodies the form and 



