156 PROTOZOA. 



slowly to protrude its root-like processes ; and foreign 

 bodies, as Diatomaceae or other minute Algae, will 

 often be found imbedded in its substance. 



Arcella vulgdris (PL XI. fig. 6) is found among 

 Confervas in ponds and ditch-water. It is contained 

 in a hemispherical shell or carapace, from the round 

 orifice of which the lobed processes are protruded. 

 The shell is covered with minute pits. 



Arcetla aculedta (PL XI. fig. 7) has the convex 

 shell furnished with spines; fig. 8 represents the 

 animal with its processes extended ; while Arcella 

 dentdta (PL XI. fig. 9) exhibits an angular or some- 

 what toothed membranous shell. Both the latter 

 species are met with in the same localities as the 

 first. 



Actinophrys sol (PL XI. fig. 12) is a very beautiful 

 and excessively delicate Rhizopod. The body is 

 spherical, and covered with very delicate and slender 

 cilia-like processes. Its movements are exceedingly 

 slow, and can only be observed by prolonged watch- 

 ing. The body appears to be reticulated, from the 

 presence of numerous vacuoles. 



Two large groups of genera and species of Rhizo- 

 poda, the animal bodies possessing the above general 

 characters, mostly with very slender processes, exist, 

 in one of which (the FORAMINIFERA) they are con- 

 tained in calcareous shells, often of elegant forms; 

 while in the other (the POLYCYSTINA) the shells are 

 siliceous or composed of flint, both kinds of shells 

 being perforated with holes. These shells, which occur 

 in the fossil state in enormous numbers, sometimes 

 forming mountain-masses, are extremely beautiful 

 objects for the microscope. 



SPON'GI^E. This class contains the Sponges, almost 

 all of which are marine and foreign, and therefore 

 not likely to come under observation in the per- 

 fect state. The substance commonly called sponge 

 is the horny skeleton of the animal, consisting usually 



