INTESTINAL RESPIRATION IN ANNBLIDS. 817 



mental fact of structure as an anus was first developed in phylogeny for the purpose of 

 getting rid of undigested remains of food ? 



(1) In the first place, when we consider the Coelenterata and Platyhelmia, it appears 

 that there is no necessary inconvenience in returning undigested matters by the mouth, 

 even in the case of animals of some considerable size. The Ophiurids indeed show us 

 that the anus may be so unimportant as to be dispensed with, even by highly organised 

 animals which at an earlier stage in phylogeny possessed this feature. It might 

 perhaps be argued that it is not size, nor even a generally high level of organisation, 

 that necessitates an anus, but length of body relatively to breadth, — that a long and 

 narrow alimentary canal, as for example in a worm-like animal, will require a posterior 

 outlet, while a short and broad nutritive cavity would not do so ; to which it may be 

 answered that many of the Turbellaiia, which possess no anus, are nevertheless much 

 elongated forms. 



To these facts of anatomy may be added a theoretical argument. In all animals 

 which possess them, the specially digestive gland or glands, whose function is to secrete 

 the juices which dissolve the nutritive portions of the food, are situated in connection 

 with the first part of the alimentary canal. The greater part of the length of the canal 

 — in those animals whose physiology has been best elucidated, and probably in others 

 also — ^is destitute of any specially active secretion ; its function is to absorb, and in 

 the case of animals with branched alimentary canal to distribute, the foodstuffs which 

 have been reduced to solution in the anterior part of the tract. The innutritions 

 portions of the food are thus separated from the nutritive portions in the anterior part 

 of the canal : if an anus exists, there is no reason why the undissolved innutritions 

 portions should not pass along the tract, in company with the dissolved matters, to the 

 anus, where they are expelled ; but an anus is by no means necessary for this purpose, 

 since, the mouth being nearer, there is little or no inconvenience in expelling the 

 undigested residue by this exit. In other words, since there is no necessity for the 

 undigested portions of the food to travel into the posterior part of the alimentary tract, 

 it is unlikely that such a fundamental fact of structure as an anus should have been 

 developed there for the purpose of allowing them an exit ; and if an anus had not 

 been independently developed for some other reason, solid food-remains would never 

 have reached the posterior part of the intestine at all. 



The original mode of expulsion of undigested food-remains in the Metazoa is by 

 the mouth. To such a degree has this been impressed on the organism, that the 

 faculty of such expulsion has never been lost. I imagine that it is within the personal 

 knowledge of everyone that what may be called the remnant of this condition persists 

 even in the highest forms — that the anterior part of the alimentary canal will still 

 spontaneously return indigestible or undigested food through the mouth, instead of 

 passing it on to be expelled through the hinder part of the tract. Reference ma)' also 

 be made to the faecal vomiting of intestinal obstruction, as showing how, on the 

 occurrence of a check to the onward passage of solid matters even low down in the 



