26 OF THE RHIZOPODA IN GENERAL :— LOEOSA. 



27. Th-e "cohtractile vesicle" does not usually project much from the surface ; though in a 

 form of ^?K<E^« allied to A.princeps, in which it has been carefully studied by MM. Clapar^de 

 and Lachmann (xxv, p. 427), it bore a very close resemblance, when in its state of greatest 

 turgescence, to that of Adino'phrys sol. When fully contracted, it disappeared entirely, but 

 soon presented itself anew as a minute vesicle in the substance of the ectosarc, which enlarged 

 little by little until it acquired its original dimensions. On its contraction, from four to eight 

 vacuoles were seen to project on different parts of the body, often at a considerable distance 

 from the contractile vesicle ;* and when these had attained a certain dimension they seemed 

 to put themselves in motion towards it, and to discharge their contents into it, — an operation 

 which is probably performed by a sort of peristaltic movement along a system of canals 

 excavated in the sarcode-substance, portions of which are distended into larger cavities by the 

 contractions of the principal vesicle, and gradually recover their original dimensions by pro- 

 pelling back to it the fluid which it injected into tliem. This kind of circulation is not 

 maintained in Amceba with the regularity that is elsewhere seen ; for MM. Claparede and 

 Lachmann observed that whilst only from half a minute to three minutes sometimes inter- 

 vened between the contractions, an interval of as much as five minutes might elapse between 

 the contractions, — a circumstance which shows how necessary it is to watch a Rhizopod for a 

 sufficiently long time, before coming to a conclusion as to the absence of a contractile vesicle. 

 It is not likely, however, that the movement of fluid which it seems to be the action of the 

 contractile vesicle to sustain, is completely suspended during so long an interval ; and as 

 tlic contractile vesicle was observed to be undergoing a continual change of bulk, it seems 

 probable that it was executing partial contractions during this period, and M'as refilled in the 

 intervals of these. 



28. A singular modification of the ordinary Amoeban type is described by MM. 

 Claparede and Lachmann (.xxv, p. 441) under the name Podostoma. This creature (which 

 was found by M. Lachmann in great abundance at Berlin, in a vessel containing Algae and 

 Infusoria) may be readily mistaken for an Amwha until it exhibits the appendages which 

 are peculiar to it ; each of these resembles at its base a short broad pseudopodium, but from 

 the extremity of this there is put forth an elongated whip-like filament, which is in continual 

 motion like the jlagellum of certain Infusoria, and which lashes the surrounding water in 

 every direction. Any foreign particle which may come into contact with this filament is 

 retained by adhesion to it, and the filament then contracts into a spiral, disappearing at 

 last in the substance of its pseudopodium, with which it becomes re-incorporated, and 

 drawing into it the attached particle, which is at first brought to the rounded extremity of 

 the pseudopodium, is then received into a spoon-shaped depression that forms itself on this, 

 and then, by a gradual deepening of this depression, finds its way into the interior of the 

 pseudopodian expansion, and thence into the soft endosarc which constitutes a sort of "general 

 cavity of the body." 



* These distensible vacuoles seem to have been mistaken by some observers for multiple contrac- 

 tile vesicles ; they have not, however, the well-definecl boundary of tliat organ ; and they do not 

 undergo the well-marked contractions which it presents at intervals. 



