APPENDIX. 



THE VASCULAR SYSTEM. 



*,* The numhers in hraclcets, 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 arc 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 GoncMlus volvox '^ and Salpina macracantha.^ 



(iii.) The lateral canals pass unexpauded 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 tlie cloaca. This has been directly observed ^ in Asplanchna prlodonta and in 

 Hydatina senta by myself, and can be easily verified. It has been suggested that a 

 return current of fresh water is dra-mi 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 viilitaris, after the outward current had driven 

 them away from it, no one else seems to have succeeded in repeating the observation.* 

 Occasionally the contents of the cloaca are driven into the intestine. Dr. Moxon has 

 seen this in Euchlanis dilatata,'^ and Dr. Semper has seen it in Trochosphcera aqiia- 

 torcaUs.^" 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. 



8. In all the three plans, given in § 1, the lateral canals sometimes appear sm-rounded 

 by a filmy, floccose substance, through which they meander (generally two on each side) 



' Dr. Semper (138) says that in Trocliosphcera aqjiatorealis there is a contractile vesicle which has 

 no connection w-ith 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 I'tcrodiiia, and in other Kotifera ; but, as 

 we differ widely here about the facts, as well as about the inferences drawn from them, 1 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 Lacimilaria socialis ; but Dr. Leydig (108) 

 says he has seen a small contractile vesicle in tliis Boliferon. Neither Pedaliun mirum, I'terudiiia 

 patina, nor P. valvata appears to have any contractile vesicle. 



* See below ; same paragraph. " Vol. i. p. 123. ' (21). 



" I have never seen B. viilitaris, which from the great size of its contractile vesicle is admirably 

 adapted for such observations. 

 » (118). " Vol. i. p. 88. 



