PHYSIOLOGY OF THE INVERTEBRATA. 25 



dig-estion, for in this order one beholds the very birth of the 

 digestive function. 



The Amoeba (Fig. 2, u) seizes its food by extending some 

 portion of its cell. The extended portion is known as a 

 pseudopodium. The pseudopodium, after seizing the particles 

 of food, retract, and the food becomes incorporated in the 

 interior of the cell, which has the property of digesting and 

 absorbing the nutritive portion of the food and ejecting the 

 non-digested portion. 



In some forms of the Protoplasta pseudopodia are extended 

 from any part of the protoplasmic cell ; whereas in others 

 (c.f/., Pamphagus) these non-differentiated prehensile organs 

 are extended from one particular region only of the cell. In 

 Arcclla and DifHugia, having an external covering or shell, 

 the pseudopodia are extended only from the single opening 

 ^3resent in each shell. 



There is another point of difference between the Grcgarina 

 and the Amoiba — namely, the latter organism has a contractile 

 vacuole. It is possible that this vacuole is in some way 

 directly connected with the function of digestion, but there is 

 no doubt that it performs more than one function, for the 

 author has shown that at times the contractile vacuole of the 

 Amccba acts as a renal organ (see later in this volume). 



As far as the function of digestion is concerned in the 

 Protopli.i.>ita, every part of the protoplasm may be made to 

 serve as a digestive cavity in enveloping the food particles. 

 " A mouth region and an anal region are marked off for each, 

 particular particle of food, but there is no mouth and there is 

 no anus." 



(t) The Foraminifera. — These complex Protozoa may be 

 looked upon as colonies of Amcehcc connected together and 

 surrounded by a complex shell. They have been divided into 

 the Perforata and the Laperforata, according to whether the 

 shell is either perforated or imperforated. In the former 

 class, the shell contains many apertures, through which the 

 pseudopodia of the particular individual dwelling within that 



