PROTOZOA I RHIZOPODA. 



43 



sarcode, sometimes very delicate, and of considerable length, 

 at other times more like finger-shaped processes; and they 

 are somewhat analogous to the little processes which are oc- 

 casionally thrown out by the white corpuscles of the blood 

 and by pus-cells. Indeed, it has been remarked by Huxley 

 that an Amceba is structurally * a mere colourless blood-cor- 

 puscle, leading an independent life.' 



Fig. 2. Morphology of Rhizopoda. a. Amoeba radiosa, showing the pseudopodia, 

 contractile vesicle, and nucleus, b. Diffluyia, with the pseudopodia protruded 

 from the anterior end of the carapace, c. Individual sponge-particles, or 

 4 sarcoids.' d. Ciliated sponge-particles of Grantia, showing the resemblance to 

 flagellate Infusorians. e. Mono-ciliated sarcoid of Spongilla (after Carter). 

 



The class Rhizopoda is divided into four orders, viz. the 

 Amoebea, the Foraminifera, the Radiolaria, and the Spongida, 

 of which the last is occasionally considered as a separate class. 



ORDER I. AMCBBEA. This order comprises those Rhizopoda 

 which are, with one or two exceptions, naked, have usually 

 short, bhmt, lobose pseudopodia, which do not anastomose with 

 one another, and contain a ' nucleus,' and one or more ' contractile 

 vesicles.'' 



The Amoeba, or Proteus-animalcule, may be taken as the 

 type, and a description of it will be sufficient to indicate the 

 leading points of interest in the order. The Amoeba {fig. 2 a) 

 is a microscopic animalcule which inhabits fresh water, and is 

 composed of gelatinous sarcode, which admits of a separation 

 into two distinct layers : an outer transparent layer, termed 

 the ' ectosarc,' and an inner, more fluid and mobile, molecular 

 layer, called the ' endosarc.' The * ectosarc ' is highly ex- 

 tensile and contractile, and is the layer of which the ' pseudo- 

 podia ' are mainly composed ; whilst the ' endosarc ' contains 

 the only organs possessed by the animal, viz. the * nucleus ' 

 and * contractile vesicle ' or vesicles, along with certain for- 

 tuitous cavities termed * food-vacuoles.' 



It is believed by some that the ectosarc is surrounded by 

 a colourless and structureless investing membrane, or cuticle ; 

 but this is denied by others. Be this as it may, there is no 

 oral cavity, and the food is merely taken into the interior of 



