PROTOZOA I RHIZOPODA. 47 



Amctbea, the Foraminifera, the Radiolaria, and the Spongida, 

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



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



I contractile vesicle, and nucleus ; b Ditflugia, with the pseudopodia protruded 

 from the anterior end of the carapace ; c Individual sponge-particles, or " sar- 

 coids ; " d Ciliated sponge-particles of Grantia, showing the resemblance to 

 flagellate Infusorians ; e Mono-ciliated sarcoid of Sfongilla. (After Carter.) 



ORDER I. AMCEBEA. This order comprises those Rhizopoda 

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

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

 one another, and contain a " nucleus" and one or more " contrac- 

 tile 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 Amceba (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, so far as has ever been certainly observed, and the 

 food is merely taken into the interior of the body by a process 

 of intussusception ; any portion of the surface being chosen for 

 this purpose, and acting as an extemporaneous mouth. When 

 the particle of food has been received into the body, the aper- 

 ture by which it was admitted again closes up, and the dis- 

 charge of solid excreta is effected in an exactly similar 

 manner. In this case, however, the area of the general sur- 

 face within which an anus may be extemporised, appears to be 



