The length of the intestinal canal in the different genera of this order varies to an extent which is curiously contrasted with the general similarity of their food. Its proportion to the length of the body is in some as 8 to I ; in others as 3 to I only. The division into two portions, and the general arrangement of both small and large intestine, is very similar to that seen in man. In all the genera a cwcurn exists, but with great variety as to length :—an increased development of this portion of intes tine, as well as of the cardiac extretnity of the stomach, being sometimes connected with a diminution in the length of the whole canal. The Apes and Gibbons possess a vermiform appendix ; but in the latter it is of very small size. The mucous membrane has villi, but no valvulte conniventes.
General remarks.—Although physiology at present scarcely pretends to interpret this various and complex development of the ali mentary canal, still some attempt at its expla nation is indispensable. For without any clue to their import, details like the preceding could hardly be recollected, far less made use of ; and would scarcely deserve to be stored up in the archives of science, much more brought forward in an essay like this. Nor, in attempting their explanation, can one be rightly charged with breaking those rules which our great countryman has laid down for the pursuit of natural knowledge. All that is necessary to such a superstructure of theory is, that, how ever slight and temporary, it should at least be founded on the known facts ; that it should in dicate something like the degree of probability assignable to its several parts; and, finally, that it should be at once yielded up, as soon as a stricter logic, or larger and more numerous facts, offer us a better explanation.
The absence of all digestive cavity is the first peculiarity which demands our notice. The few genera in whom this rare condition has been found all offer the greatest simplicity of structure; and further agree in the fact that they are parasitic :—i. e. that they derive their nutriment from the juices of another animal, to whose body they are attached. IIence we need not scruple to assign this apparent defi ciency of the digestive organ, partly to the pre vious elaboration of a highly nutritious animal food, partly to the simplicity of the various tissues which are destined to be nom ished by it. But can we therefore say, that the function of digestion is absent, or — what would be nearly equivalent to such an assertion — that it is reduced to a mere phy sical absorption ? Probably not. For, as regards the general de velopment of the animal series, comparative 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 metn 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 in the stomach. And the sarne 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.