STOMACH AND INTESTINE. 



305 



anatomy conclusively shows that the fusion 

 of certain structures by no means implies 

 the absence of their several functions ; 

 while a history of the development of each 

 individual would equally establish that, though 

 the embryo at a certain stage of life is quite 

 devoid of a digestive cavity, it is nevertheless 

 nourished by materials which have been pre- 

 viously set apart from the substance of the 

 parent. And just as it must doubtless effect 

 some change in these materials, in order to 

 assimilate them to its own various textures, 

 so it is evident that such a change, however 

 slight, probably represents what is as much a 

 digestive as an absorptive act: a digestion in 

 which the absence of many of the ordinary 

 agents is sufficiently accounted for by the mini- 

 mum of waste which this food supplies, and the 

 minimum of change which it has to undergo. 

 Now some of these parasitic genera are also con- 

 nected by the circumstance, that the anenterous 

 condition probably forms but a stage of their 

 development ; so that the process of time, 

 or their transplantation to a more congenial 

 dwelling, would often convert them into 

 animals possessing an alimentary canal. Of 

 such creatures we might therefore vaguely say, 

 that they retain the low digestive development 

 of an early ovum; or, in other words, that they 

 are themselves the partially developed embryos 

 of a very simple organization. That, with 

 such a simple structure, they should effect 

 such a complex function, is surely not one whit 

 more extraordinary than what appears to be 

 the case in the action of every ordinary cell ; 

 which is what it is liver, kidney, or the like 

 by virtue of powers that its mere structure 

 will not explain powers that enable it to 

 attract and retain certain materials, to re- 

 linquish or dismiss others, or even to effect a 

 definite metamorphosis in its own chemical 

 ingredients. 



The simplest form of the digestive organ may 

 be seen in the hydriform Polyp, as a cavity of 

 the body, in which the food undergoes a kind* 

 of solution. The agent of this process is 

 doubtless a fluid which exsudes from the mem- 

 branous walls of the cavity. But as these are 

 also the parietes of the body, it is to the 

 latter that we must probably refer the origin of 

 the solvent. That harmless inversion of the 

 whole animal, which Trembley was able to effect, 

 strengthens such a conjecture. Nor is it impos- 

 sible, that the poison of the tentacles is itself 

 but a more concentrated form of the gas- 

 tric fluid. In any case, one cannot avoid 

 suspecting that, in this animal, the alimentary 

 solvent has some very simple chemical rela- 

 tion to the organism generally. The more so 

 that, although it acts upon the swallowed prey 

 with the greatest energy and rapidity, the 

 tentacles of the animal itself, which are often 

 closely entwined around the hapless victim, 

 are quite unaffected by even a prolonged stay 



* For the sense in which we are to understand 

 the word solution as applied to this process, see 

 the remarks upon the action of the gastric juice at 

 p. 337. 



Supp. 



in the stomach. And the same impunity ex- 

 tends to another animal of its own species 

 which may have been swallowed while tena- 

 ciously clinging round the prize * that both 

 are disputing. 



It is usual to call such a simple digestive 

 cavity a "stomach." But though the etymo- 

 logy of the term quite allows of its being 

 thus applied, still the definite character of 

 this organ in the higher animals seems to sug- 

 gest that we should either restrict its applica- 

 tion, or recollect the doubtful meaning which 

 it acquires by such an extended use. When- 

 ever an organ of this kind appears to effect 

 the solution of substances which pertain to 

 the albuminous groupf, it is entitled to rank as 

 a true stomach. But in proportion as this fact 

 is uncertain or improbable, the name becomes 

 a vague designation, which ought never to be 

 made use of without recollecting what it 

 really means a mere receptacle of food. In 

 the instances before us, such a receptacle 

 probably represents, not only the stomach of 

 the higher animal, but a fusion of this with 

 the succeeding J portions of the tube, and with 

 all the accessory organs of digestion, such as 

 the liver, pancreas, &c. And just as such 

 simple cavities import more than a mere 

 gastric function, so conversely we might find 

 others bearing the same name, which com- 

 plicate a fully developed alimentary canal, 

 and thus imply less. These, though called 

 stomachs, are probably mere crops. 



A complex digestive organ might at first 

 sight seem to be the natural antithesis of the 

 preceding. But though complexity forms a 

 useful subjective contrast, without which we 

 could indeed hardly conceive of simplicity 

 still, as already hinted, instead of a progressive 

 evolution, corresponding to a gradual and suc- 

 cessive advance of development, the alimentary 

 canal rather offers a variety of deviations. And 

 most of these deviations appear to result from 

 causes, the number and intricacy of which 

 is such as to defy all analysis. We shall 

 therefore only enumerate those, the influence 

 of which seems to be most direct and im- 

 portant. 



1. It is scarcely necessary for us to dwell 

 upon that advance of development, and gradual 

 increase of complexity, which the reader must 

 have observed in the preceding sketch. He 

 has seen how, in progressing from the low- 

 est Infusory to the highest Mammal, a simple 

 excavation first became a membranous canal ; 

 how it then acquired an additional orifice; 

 an organ of mastication ; a salivary appa- 

 ratus ; a stomachal dilatation ; a subdivi- 



* It is interesting to notice that differences of 

 development, such as are obviously almost tanta- 

 mount to diversity of species, appear to remove all 

 barrier to this solvent action. Thus the Polypiform 

 Medusa devours and digests its Infusory-like younger 

 brethren. 



t See p, 335. 



j The term " chylific stomach," sometimes made 

 use of by comparative anatomists, seems especially 

 to demand such a caution. For \ve need scarcely 

 point out that, in the higher animals, at any rate, 

 this organ does not "make chyle." 



