28 THE INVERTEBRATA 



between the gametes, and that of the malaria parasite (Fig. 75) and 

 its relations is anisogamy between a hologamete and a merogamete. 



Syngamy, whether isogamous or anisogamous, nearly always is exo- 

 gamous, that is, takes place between the offspring of different parents. 



Since the male and female gametes are usually formed by distinct 

 parents, sex may be said to exist among protozoa, but it is rarely that 

 (as in the sporozoon Cyclospora, etc.) the sexes may be distinguished 

 by other features. In many of the Telosporidia (e.g. Monocystisy 

 Fig. 78) sexual cojigress may be held to occur, in that individuals, 

 male and female in cases of anisogamy, apply themselves together at 

 the time of gamete formation, and their gametes unite each with one 

 from the other parent. Hermaphroditism appears in the Ciliophora^ 

 (except the Opalinidae and Chonotricha). Here congress (Fig. 26) 

 takes place between two individuals (conjugants) in each of which the 

 meganucleus (see above) disintegrates, and the micronuclei divide to 

 form a number of nuclei — perhaps a reminiscence of the formation 

 of numerous merogametes. All but one of these nuclei disappear, 

 and the survivor divides to form a male pronucleus, which passes over 

 into the partner, and a female pronucleus which, in possession of the 

 cytoplasm of the parent, awaits the arrival of the male pronucleus of 

 the partner. Fusion now takes place between the male and female 

 pronuclei in each of the pair of conjugants, the latter separate, and 

 by the division of their zygote nuclei mega- and micronuclei arise. 

 Two hermaphrodites have formed each a male and a female gamete 

 and cross-fertilization has taken place.^ In the Vorticellidae (Fig. 26 B) 

 the individuals which enter into congress differ, one being of the 

 ordinary size and fixed, the other small and free-swimming. The 

 smaller arises from an ordinary individual, as a bud or by repeated 

 fission. After reciprocal fertilization of the type just described, the 

 smaller partner perishes, its endoplasm being sucked into the larger. 

 This curious simulation of sexual dimorphism by hermaphrodites 

 occurs in a less marked form in other ciliates. 



A remarkable process known as autogamy, in which a nucleus 

 divides into two which after maturation immediately reunite, occurs 

 in Actinophrys and Actinosphaerium (see p. 78), and possibly in some 

 other cases. 



Parthenogenesis is known to occur in members of at least three of 

 the four classes of the phylum. The clearest case is presented by 

 Actinophrys, when gametes which have failed in attempt at cross- 



1 Actinophrys (p. 78) may be said to be hermaphrodite, and so perhaps 

 are many of the Radiolaria. But it is not certain that the "gametes " of this 

 group are not parasitic dinoflagellates. (See p. 75.) 



2 Occasionally {CoUinia, Detidrocometes) the conjugants also exchange 

 halves of their meganuclei. The latter, however, always disintegrate. 



