142 PHYLUM PROTOZOA — tHE SIMPLEST ANIMALS 



cells. This difference between Metazoa and Protozoa in 

 their modes of multiplication is a consequence of the 

 difference between multicellular and unicellular life. Each 

 part of a divided Protozoon is able to live on, and will 

 itself divide after a time, whereas the liberated spermatozoa 

 and ova of a higher animal die unless they unite. 



By sexual reproduction we mean — (a) the liberation of 

 special reproductive cells from a " body," and (b) the 

 fertilisation of ova by spermatozoa. As Protozoa have 

 no " body " — though the beginnings of one are seen in 

 the colonial forms — they cannot be said to exhibit sexual 

 reproduction in the first sense (a), yet many of them^ 

 (especially the Sporozoa) give origin by division to special 

 reproductive cells. And although many Protozoa can live 

 on, dividing and nujltiplying, for prolonged periods without 

 the occurrence of anything like fertilisation, processes 

 corresponding to fertilisation are of general occurrence. 

 For in many of the Protozoa there occurs at intervals a 

 process of " conjugation *' in which two individuals unite 

 either permanently or temporarily. This is an incipiently 

 sexual process ; it is the analogue of the fertilisation of an 

 ovum by a spermatozoon. In many cases, moreover, 

 there is a difference between the two conjugates, analogous 

 to the difference between ovum and spermatozoon. 



(i) It is one of the recurrent phases in the Hfe-history of some of the 

 simplest Protozoa (Proteomyxa and Mycetozoa) (see Fig. 59), that a 

 number of amoeboid units flow together into a composite mass, which 

 has been called a " Plasmodium.'" 



(2) It is known that more than two individual Sporozoa and other 

 forms occasionally unite. To this the term " multiple conjugation " 

 has been applied. 



(3) Commonest, however, is the union of two apparently similar 

 individuals, either permanently, so that the two fuse into one, or 

 temporarily, so that an exchange of material is effected. Permanent 

 conjugation has been observed in several Rhizopods, Infusorians, and 

 Sporozoa. Temporary conjugation is well known in not a few ciliated 

 Infusorians, and it is possible that a curious end-to-end union of certain 

 vSporozoa is of the same nature, or it may be of the nature of a 

 " Plasmodium " formation. The formation of small spores (gametes) 

 which conjugate is not uncommon. 



(4) There are some cases where one of the conjugating individuals 

 is larger and less active than the other. Thus in Vorticella, a small 

 free-swimming form unites and fuses completely with a stalked indivi- 

 dual of normal size. This " dimorphic conjugation " is evidently 



