APPENDIX. 137 



ends, or are they closed ? Do tliey contain an undulating membrane, or are tlieir inner 

 surfaces furred with minute cilia? Does a current pass through them (supposing them 

 to be open funnels) toicards 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 

 having no outlet save at the contractile vesicle ? Is the substance surrounding the 

 lateral canals a glandular secreting substance, or a mere mechanical sujiport for the 

 canals '? Does the contractile vesicle till 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 tlieir receiving any answers that will meet with general 

 acceptance, for on all these points the best observers disagree : I sliall, 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 oi^inion 

 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 a necessity. 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) ahnost 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- 

 ever, 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 fluid 

 passes into the lateral canals, to be discharged through the cloaca, then we arc met with the difliculty 

 that this supposition would imply the freipient 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 sucll 

 a highly organised product as blood. 



