260 FRESH-WATER RHIZOPODS OF NORTH AMERICA. 



and a clearer peripheral layer. The latter, as seen in the various figures 

 of pi. XLI, consists of a single stratum of large clear vesicles or vacuoles, 

 forming mostly short six-sided columns, resting by their narrower end on an 

 interior ball of smaller polyhedral vesicles, and with the opposite end free 

 and convex, and contributing to the general surface of the body. The 

 superficial vesicles are nearly uniform, but not unfrequently vary, and 

 sometimes a pair of vesicles of more uniform diameters substitute the ordi- 

 nary short columnar vesicle. The interior vesicles, besides being in general 

 smaller than the exterior ones, are more regularly polyhedral and of greater 

 uniformity in their several diameters. 



The vesicles are composed of a thin, delicate, protoplasmic layer, con- 

 taining apparently a more liquid and homogeneous protoplasm within. 

 They appear to be held together and invested with a more granular proto- 

 plasm with diffused oil molecules. A thicker stratum of this material ap- 

 pears to define the peripheral layer of vesicles from the deeper mass, and a 

 greater proportion also appears to extend between the vesicles of the latter. 

 From this more granular protoplasm investing the body of the animal, that 

 of the pseudopodal rays mainly has its origin. 



In the peripheral layer of vesicles there are usually to be detected two 

 contractile vesicles, situated at opposite poles of the body, as seen in figs. 

 1, 2, 3, fl, b. At one moment they may exhibit no difference in appearance 

 from the contiguous vesicles; but, from time to time, one or both may be 

 seen slowly to enlarge, pressing on the surrounding vesicles, and rising as 

 a clear hemispherical bubble above the surface of the body. See figs. !-5, 

 7, a. On reaching the full degree of expansion, they rather abruptly col- 

 lapse, and expel the liquid contents, causing a sinking of the wall of the 

 vesicle, and producing a temporary concave depression on the surface of 

 the body, as seen in figs. 1, 2, b. The collapse of the vesicles is sufficiently 

 strong to give a visible shock to the body of the animal. Shortly after the 

 collapse, the contractile vesicle reappears in the same place. 



According to Stein, Carter, and other authorities, Actinosphcerium eicli- 

 hornii contains many nuclei, large individuals having a hundred or more. 

 They occupy a position in the superficial part of the interior mass of vesi- 

 cles, beneath the peripheral layer of larger ones, enveloped in the same 

 kind of finely granular protoplasm. Ordinarily, they are invisible without 

 the application of chemical reagents, or at most they are sometimes b.irely 



