APPENDIX. 137 
ends, or are they closed? Do they contain an undulating membrane, or are their inner 
surfaces furred with minute cilia? Does a current pass through them (supposing them 
to be open funnels) towards the lateral canals, as it seems to do, or in the opposite 
direction? Are the lateral canals open at their upper ends, or are they blind passages 
haying no outlet save at the contractile vesicle? Is the substance surrounding the 
lateral canals a glandular secreting substance, or a mere mechanical support for the 
canals? Does the contractile vesicle fill itself by drawing up fresh water through the 
cloaca, or is it filled by fluid passing into it from the lateral canals ? 
I do not know how these questions are to be answered with any approach to certainty, 
and I have no expectation of their receiving any answers that will meet with general 
acceptance, for on all these points the best observers disagree: I shall, therefore, do no 
more than state, in the following paragraph, the view of those who consider the vascular 
system to be an excreting one; and leave to my colleague the advocacy of the opinion 
which he has long held, viz. that the system is mainly branchial, with, possibly, a sub- 
ordinate excreting function. 
7. The perivisceral fluid is in part! derived from the products of digestion which pass 
by endosmose through the cellular walls of the stomach ; and it is out of this fluid that 
the various organs are repaired, and at its expense that the animal moves and grows. 
This growth, repair, and action change the constitution of parts of the perivisceral fluid, 
and render an excreting organ anecessity. The vascular system is this excreting organ ; 
and, indeed, no other has ever been suggested as having an excreting function. The 
lateral canals with their floccose investments, or the vibratile tags,” or both, are the 
excreting vessels; while the part played by the contractile vesicle is one probably of 
storage and discharge : for the contractile vesicle varies extremely in size and frequency 
of action in different Rotifera, and in some is altogether absent. 
The oxygenation of the perivisceral fluid, both in males and females, probably takes 
place at the fore part of the head, where the skin is never loricated, but appears to be 
thin; and, where, too, it is possible that there may be definite spots, covered with deli- 
cate membrane, so as to take advantage of the constant rush of water, drawn to the 
head by the ceaseless action of the cilia. 
8. Now it is obvious (from § 6) that the above explanation (given in § 7) of the 
vascular system, rests on a number of assumptions which it is impossible to verify. But 
then as much, I think, may be said of the explanation that would make the function of 
this system a branchial one, or a combination in various degrees of both. 
! Leydig is of opinion (110) that water passes by endosmosis into the body cavity. This, indeed, 
seems probable; for indigo-coloured water when swallowed (e.g. by R. vulgaris) almost instantly 
imparts a blue tint to the thick cellular walls of the stomach up to their outmost boundary. It can 
hardly be supposed that it goes no further, if the products of digestion do. It seems unlikely that the 
inner walls of these stomach-cells should be pervious to the products of digestion, and to water, alike ; 
and that the outer walls should be pervious to the former, and impervious to the latter. It has, how- 
eyer, been objected, that we never see the indigo-coloured water in the perivisceral fluid. But it is 
hardly to be expected that we should. When we look at the blue stomach-walls, we are looking at a 
colour produced by a depth of solution equal to that of one or two thick cells; whereas the coloured 
fluid, oozing out through the stomach-walls, would be presented to our eyes in films of almost 
infinitesimal thinness ; which would never be suffered to accumulate and so become visible, but would 
be at once broken up and lost, by the constant motion of the perivisceral fluid. Besides the blue tint 
after a time disappears from the cells. It seems more likely that this is due to the indigo-solution 
continuing its course through the cells into the body cavity along with the products of digestion, than 
to its parting company with these latter at the outer wall, and then alone reversing its course, and 
returning into the stomach. 
* If the vibratile tags be supposed to be open ciliated funnels, through which the perivisceral fiuid 
passes into the lateral canals, to be discharged through the cloaca, then we are met with the difficulty 
that this supposition would imply the frequent discharge of a fluid analogous to blood. But, on the 
other hand, if it were admitted that, in the perivisceral fluid, the products of digestion are largely 
diluted with water (see previous note), the force of this objection would be much weakened ; for the 
supposed difficulty would be mainly due to our having applied, to so simple a fluid, the name of such 
a highly organised product as blood. 
