PROTOZOA. 







these movements is to be sought in the contractility of the surround- 

 ing portions of protoplasm (Fig. 1). 



A pulsating space, the contractile vacuole, is not unfrequently to 

 be found in the protoplasm, e.g., Difflugia, Actinophrys, Arcella 

 (Fig. 2). Nuclei are usually to be made out, but there are forms 

 in Avhich no trace of a nucleus has yet been found. In such cases 

 either our methods of observation are faulty, or the protoplasm 

 of the nucleus is not yet differentiated as a separate structure 



(the Monera of E. Haeckel), or we 

 have to do with a transient, non- 

 nucleated stage in the life-history. 



The protoplasm usually secretes 

 silicious or calcareous structures, 

 either as fine spicula and hollow 

 spines, which are directed from the 

 centre to the periphery in regular 

 order and number, or as lattice- 

 work chambers (Radiolaria), which 

 often bear points and spines, or 

 finally as single and many-chambered 

 shells with, in some cases, finely 

 perforated walls (Foraminifera) and 

 larger opening. Through this 

 last (Fig. 4), as well as through the 

 countless pores of the small shells 



(Fig. 3), the slender threads of sarcode pass out to the exterior 

 as pseudopodia, changing without intermission in form, size, and 

 number, and often joining themselves together in delicate networks. 

 (Figs. 3, 4.) 



The pseudopodia, by their slow contraction, afford a means of 

 locomotion, while they also serve for the taking up of nourishment 

 by surrounding and transporting into the interior of the body small 

 vegetable organisms as Bacillaria. Among the shell-bearing forms, 

 the reception and digestion of food takes place outside the shell in 

 the peripheral threads and networks of sarcode ; for each spot on the 

 surface can for the time being assume the functions of mouth, and 

 also of anus, by rejecting the undigested remnants. While the 

 power of emitting pseudopodia is characteristic of the form in which 

 the Gymnomyxa are usually met with, it is usual to find a stage in 

 their life-history in which locomotion is effected by means of flagella. 

 To such flagellated forms the term mastigopod has been applied, 



pulsating vacuole. 



