172 PRINCIPLES OF ANIMAL BIOLOGY 



in each. In Stylonychia and many other forms the ciliary apparatus 

 is developed afresh, sometimes before fission is completed. In forms 

 which have both macro- and micronuclei, both nuclei elongate and finally 

 divide, a half going to each new individual. After the separation into 

 two individuals regeneration is completed and each individual grows in 

 size. In the protozoan parasite Opalina ranarum division of the nucleus 

 proceeds until many nuclei, even as many as a hundred or more, are 

 produced, then the cytoplasm divides by successive binary fissions to 

 form as many little cells as there were nuclei. Encystment of the small 

 individuals frequently follows these divisions. In Paramecium, when 

 well fed, fission occurs once in about 16 to 24 hours. 



As a method of reproduction fission occurs in certain of the worms, 

 as the triclad Turbellaria and some species of nemerteans. 



Sporulation. — Many species of Protozoa produce large numbers of 

 minute individuals by a process of internal or external budding or by 

 multiple fission. This process is commonly called sporulation. This 

 is one of the methods of reproduction in the Sporozoa, a class of Protozoa 

 parasitic in the cells, tissues and cavities of many of the metazoa. Ex- 

 amples are the gregarines, coccidians and malarial organisms. The 

 gregarine Monocystis is rarely absent from the seminal vesicles of the 

 earthworm. Other gregarines occur in the cells and lumen of the intestine 

 of insects and their allies, the arthropods. Coccidians occur in the cells 

 and lumen of the intestine and other organs of arthropods, other inver- 

 tebrates, and vertebrates. 



The method of sporulation as it occurs in Coccidiwn schuhergi is 

 illustrated in Fig. 132, IV-VII. This parasite occurs in the intestine 

 of Lithohius forficatus, a centipede. A part of the life cycle {11-VII, 

 XIa-XIc and Xlla-XIIc) occurs within the cells of the intestine, part 

 of it (Xlld-XIX) free in the intestine of Lithobius and part of it (in 

 a resting condition) outside the body of the host. When the oocyst 

 (XX) is swallowed by Lithobius the cyst opens and rainute ^wrozoites 

 emerge. They enter cells of the intestine (77) where they grow (777, IV) 

 and finally by multiple division of the nucleus and fragmentation of 

 the cytoplasm (V, VI, VII) a large number of minute motile indi- 

 viduals [VIII, IX, X) are formed. These minute individuals may be 

 called merozoites. This latter part of the hfe history (II-VII) is wholly 

 asexual and is sporulation, the rest of it has to do with the formation of 

 gametes (sexual individuals). 



Limits of Asexual Reproduction. — Asexual reproduction occurs only 

 among the lower forms of animal life. It never occurs among vertebrate 

 animals, although it docs occur among the ascidians which are placed in the 

 phylum to which vertebrates are also assigned. It also does not occur in 

 the following phyla of invertebrates: the Nemathehninthes (round- 

 worms), the Rotifera (rotifers), Echinodcrmata (starfishes, sea-urchins, 



