Chapter 17 

 Class 2 Sarcodina Hertwig and Lesser 



THE members of this class do not possess any thick pellicle 

 and, therefore, are capable of forming pseudopodia (p. 49). 

 The term 'amoeboid' is often used to describe their appearance. 

 The pseudopodia serve ordinarily for both locomotion and food- 

 capturing. The peripheral portion of the body shows no structural 

 differentiation in Amoebina, Proteomyxa, and Mycetozoa. Internal 

 and external skeletal structures are variously developed in other 

 orders. Thus, in Testacea and Foraminifera, there is a well-devel- 

 oped test or shell that usually has an aperture, through which the 

 pseudopodia are extruded; in Heliozoa and Radiolaria, skeletons of 

 various forms and materials are developed. 



The cytoplasm is, as a rule, differentiated into the ectoplasm and 

 the endoplasm, but this differentiation is not constant. In Radio- 

 laria, there is a perforated membranous central capsule which marks 

 the border line between the two cytoplasmic regions. The endoplasm 

 contains the nucleus, food vacuoles and various granules. The ma- 

 jority of Sarcodina are uninucleate, but species of Foraminifera and 

 Mycetozoa are multinucleate in certain phases during their develop- 

 ment. In the family Paramoebidae, there occurs a peculiar secondary 

 nucleus. 



The Sarcodina are typically holozoic. Their food organisms are 

 Protozoa, small Metazoa and Protophyta, which present themselves 

 conspicuously in the cytoplasm. The methods of ingestion have al- 

 ready been considered (p. 97). One or more contractile vacuoles 

 are invariably present in forms inhabiting the fresh water, but absent 

 in parasitic forms or in those which live in the salt water. 



Asexual reproduction is usually by binary (or rarely multiple) 

 fission, budding, or plasmotomy. Definite proof of sexual reproduc- 

 tion has been noted in a comparatively small number of species. 

 Encystment is common in the majority of Sarcodina, but is unknown 

 in a few species. The life-cycle has been worked out in some forms 

 and seems to vary among different groups. The young stages are 

 either amoeboid or flagellate, and on this account, it is sometimes 

 very difficult to distinguish the Sarcodina and the Mastigophora. 

 In some forms the mature trophic stage may show an amoeboid or 

 flagellate phase, owing to differences in environmental conditions. 



The Sarcodina are divided into two subclasses as follows : 



With lobopodia, rhizopodia, or filopodia . . Subclass 1 Rhizopoda (p. 418) 

 With axopodia Subclass 2 Actinopoda (p. 505) 



417 



