14 
Transactions of the Society. 
I do not see bow we can decline to accept such a precise state- 
ment, made as it is by an expert ; and, if we admit its accuracy, how 
can we escape from the conclusion, that the fluid, which distends the 
contractile vesicle, is probably little else but water drawn in through 
the cloaca ? 
And, should this explanation prove to be correct, there would then 
be no difficulty concerning the quantity of fluid, so frequently expelled 
from the contractile vesicle. The lateral canals and vibratile tags 
might then he considered, as before, to form a secreting organ, whose 
secretion did indeed enter the vesicle, but was there so diluted, with 
water drawn up from the cloaca, that it in no way injured that water 
in its office of aerating the perivisceral fluid through the delicate wall 
of the vesicle.* 
To sum up, then, I think that of the various explanations offered 
of this perplexmg system of organs, the most probable one is that 
it is an excreto-respiratory one ; the contractile vesicle performing 
the function of respiration, and the lateral canals that of secretion ; 
and that these functions remain unaltered, whether the lateral canals 
are united to the vesicle or not. It is evident, however, that to place 
this explanation beyond doubt, Cohn’s experiment should be repeated 
several times ; Trochosphsera should be thoroughly re-examined ; a 
record should be kept of the rate at which the vesicle contracts in 
various species ; and an estimation made in such case of the ratio of 
its volume to that of the perivisceral fluid. 
The study of such minute details, no doubt, is dry, and I am 
afraid that my recital of them has proved wearisome ; but then 
natural history, to a large extent, is a study of minute details, which, 
indeed, must always be its sure foundation. And yet this study has 
its compensations ; for while engaged in it, laying the foundations of 
such a work as man generally raises — solid perhaps, certainly iormal, 
and probably heavy — I became aware of the silent growth, on the 
same foundation, of a palace of delight, into which I could enter at a 
wish, and leave the world behind me. Here could I roam through 
pleasant chambers, rejoicing in their treasures of memory — in their 
store of early fancies glittering in the light of happy youth — and in 
strange prizes, won in dear companionship, among all the charms of 
cliff, combe, sea, and sunshine. Here, too, were corridors of half- 
farmed thoughts, stretching out into that enchanted region where a 
few grains of fact, like a drop or two of a compressed gas, expand into 
clouds of ideas, hazy, yet tinted with the hue of hope — clouds that 
* Now the probability of this theory being true is strengthened by the case of 
Trochosphxra xquatorialis For Semper distinctly states that in Trochosphxra the 
lateral canals are entirely detached from the contractile vesicle; and that, instead of 
terminating on its surface, they both pass below it to the cloaca, and open just at the 
cloacal aperture. With such an arrangement of the parts, it is hardly possible to 
suppose that the contractile vesicle is distended by fluid discharged from the canals. 
So here, too, we seem driven to the conclusion, that water drawn through the cloaca 
is the principal agent in the distention of the vesicle. 
