RHIZOPODA 9 



rhizopods, called Amoeba, present a globular form when 

 contracted, but can extend portions of their substance ("sar- 

 code") like roots, and use them to draw along the rest 

 of the mass, like the feet or tentacles of polyps, whence 

 the name of the class. These root-like processes can also 

 attach themselves to foreign particles, and draw them into 

 the " sarcode," or substance of the body, where the soluble 

 organic part, so " intus-suscepted," may be assimilated, the 

 insoluble part being extruded. A solid hyaline corpuscle or 

 nucleus is commonly discernible in the interior of the Amoeba, 

 sometimes accompanied by one or more clear contractile 

 vesicles. When the productions of the sarcode are numerous, 

 filiform, and seemingly constant, radiating from all parts of 

 the body, the rhizopod presents the characters of Actinophrys. 

 When the tentacles are produced from only one extremity of 

 the body we have the genus Pamphagus. When such a 

 rhizopod is enclosed in a membranous sac it is a Difflugia ; if 

 the sac be discoid with a slit on the flat surface for the protru- 

 sion of the tentacles, it is an Arcella. In other rhizopods the 

 sac is calcified, or becomes a "shell," which is sometimes simple, 

 but usually consists of an aggregate of chambers, inter-commu- 

 nicating by minute apertures, whence the name Foraminifera 

 given to the testaceous rhizopods. These chambers grow by 

 successive gemmation from a primordial segment, sometimes 

 in a straight line, more commonly in a spiral curve ; and each 

 segment so developed has its own shelly envelope. As, how- 

 ever, they are organically connected, the whole seems to form 

 a "chambered" or " polythalamous" shell. The last-formed 

 segment is usually distinguished by the very long, slender, 

 pellucid, colourless, contractile filaments which have suggested 

 the name " Ehizopods" for the class. But, in the Foraminifera, 

 both the outer w T all and the septa of the compound shell are 

 perforated by minute apertures, through which either connect- 

 ing or projecting filaments of the soft organic tissue can pass. 



