pee By ND eXe 
THE VASCULAR SYSTEM. 
*," The numbers in brackets, as (138), refer to the memoirs in the Bibliography, pp. 140-142. 

1, This system of vessels, in its usual form, has already been described in vol. i. p. 8. 
There are three! principal varieties of it, including that already given above. 
(i.) The lateral canals open into a contractile vesicle, which discharges itself into 
the cloaca. This is the ordinary plan. 
(ii.) Each of the lateral canals ends in an expanded portion which dilates, and con- 
tracts, and discharges into the cloaca. This doubling of the contractile vessel is to be 
found, among others, in Conchilus volvox ? and Salpina macracantha.* 
(iii.) The lateral canals pass unexpanded directly into the cloaca, and the contractile 
vesicle is absent.* 
2. It is probable that the contractile vesicle is filled by a fluid flowing into it 
through the lateral canals, and it is certain that it usually ° empties itself outwards 
through the cloaca. This has been directly observed ® in Asplanchna priodonta and in 
Hydatina senta by myself, and can be easily verified. It has been suggested that a 
return current of fresh water is drawn up by the expanding contractile vesicle through 
the cloaca ; but no one has seen any appearance of this in the cloaca itself; and though 
Dr. Cohn? thinks that he saw a return current draw particles of carmine towards the 
opening of the cloaca of Brachionus militaris, after the outward current had driven 
them away from it, no one else seems to have succeeded in repeating the observation.8 
Occasionally the contents of the cloaca are driven into the intestine. Dr. Moxon has 
seen this in Huchlanis dilatata,® and Dr. Semper has seen it in Trochosphera equa- 
torealis.!° In each case it was effected by closing the aperture of the cloaca and open- 
ing that of the intestine simultaneously ; but this is not the usual action, and (as Dr. 
Moxon suggests) seems only to be a method of obtaining a natural enema for a clogged 
intestine. 
3. In all the three plans, given in § 1, the lateral canals sometimes appear surrounded 
by a filmy, floccose substance, through which they meander (generally two on each side) 
1 Dr. Semper (138) says that in Trochosphera equatorealis there is a contractile vesicle which has 
no connection with the lateral canals: if this is really the case, it would be unique. Mr. Gosse has 
described, p. 138, another variety of the vascular system in Pterodina, and in other Rotifera; but, as 
we differ widely here about the facts, as well as about the inferences drawn from them, I have (for the 
sake of brevity and clearness) omitted this variety from my account. 
* Vol. i. p. 90. 3 Vol. ii. p. 85. 
‘ Professor Huxley (91) states that this is the case in Lacinularia socialis; but Dr. Leydig (108) 
says he has seen a small contractile vesicle in this Rotiferon. Neither Pedalion mirum, Pterodina 
patina, nor P. valvata appears to have any contractile vesicle. 
5 See below; same paragraph. § Vol. 1, p. 123. 7 (21). 
8 T have never seen B. militaris, which from the great size of its contractile vesicle is admirably 
adapted for such observations. 
» (118). 10 Vol. i. p. 88. 
