i8 



THE INVERTEBRATA 



surrounding medium, either in the habitat in which encystment takes 

 place or in those encountered in the course or distribution. They 

 may on the other hand be resting cysts, which enable the organism 

 to proceed undisturbed with digestion or photosynthesis or by quies- 

 cence to conserve its energy during starvation. Cysts which subserve 

 reproduction may hegamocysts, in which union of gametes takes place 



^kar. 



Fig. 1 8. Nuclei of Protozoa. A, Polystomella crispa. B, Amoeba proteus. 

 C, Actinosphaerium eichhorn. D, Naegleria bistadialis. E, Polytoma uvella. 

 F, Ceratium fiisus. G, Stentor coeruleus. All highly magnified, to various 

 degrees. After various authors, cen.} possible centriole; kar. karyosome, 

 containing a centriole in D; nu.' nucleoli or amphinucleoli. 



(gregarines, Figs. 76-78), oocysts, containing a zygote, or sporocysts 

 containing several small individuals produced by fission. The oocyst 

 frequently becomes a sporocyst by fission of the zygote. Reproductive 

 cysts are often also resistance cysts. 



The nuclei of the Protozoa (Fig. 18) vary greatly in structure. 



