50 MANUAL OF ZOOLOGY. 



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 nlainly 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 splid excreta is effected in an exactly similar manner. 

 In this case, however, the area of the general surface within 

 which an anus may be extemporised, appears to be more 

 restricted, and to comprise a portion only of the body 

 (" villous region"). 



The "nucleus" is a solid granular body, one or more of 

 which is present within the endosarc of every Amoeba, but 

 its function is not known with any certainty. The "con- 

 tractile vesicles" are cavities within the endosarc, of which 

 ordinarily one only is present in the same individual, though 

 sometimes there are more. In structure it is a little cavity or 

 vesicle filled with a colourless fluid apparently derived from 

 the digestion, and exhibiting rhythmical movements of con- 

 traction (systole) and dilatation {diastole). In some cases radi- 

 ating tubes are said to have been seen proceeding from the 

 vesicle at the moment of contraction. Regarded functionally, 

 the contractile vesicle must be looked upon as a circulatory 

 organ, and it offers therefore the most rudimentary form of a 

 vascular system with which we are as yet acquainted. 



Besides these proper organs, the endosarc usually contains 

 clear spaces, which are called " vacuoles," or, more properly, 

 " food-vacuoles." These spaces are of a merely temporary 

 character, and are simply produced by the presence of par- 

 ticles of food, usually with a httle water taken into the body 

 along with the food. 



