424 BIOLOGY OF THE PROTOZOA 



karyon divides three times and eight daughter nuclei are formed 

 which become the nuclei of eight sporozoites. In each sporocyst, 

 therefore, there is a possibility of as many zygotes and sporoblasts 

 as there are gametes formed by one of the original gregarines. The 

 parasites are passed out of the intestine with the feces and further 

 development is inhibited until the sporoblasts are eaten by another 

 host. 



A more complex, but still simple life history involves a change of 

 hosts. The genus Forospora appears to be represented by several 

 species which pass their trophic stages in the digestive tract of 

 Crustacea and their sexual stages in mussels. Porospora gigantea 

 grows to an enormous size (up to 16 mm.) in the lobster (Homarvs 

 sp.) where it apparentl\' lives for a long periofl. Ultimately and 

 either in association or individually', it becomes spherical and forms 

 a cyst-like ball with a diameter of 3 to 4 mm. The ball then divides 

 into many gametocytes each with a diameter of from 5 to 8^t, and 

 each gametoc^'te forms gametes which are arranged radially about 

 a central residual bod\'. The gametes are very small (.3ju long by 

 l;u in diameter) and pass out with the feces into the water with 

 which they enter the digestive tract of the mussel {Mytilus edulis) 

 where they unite to form zygotes. Each zygote forms a single 

 sporozoite which is liberated in the gut of the lobster. 



The Schizogregarinida are more complicated through the intro- 

 duction of an asexual reproductive phase in the life history leading 

 to spread of the infection in the same host. Under the term "multi- 

 plicative reproduction" Doflein distinguishes this phase from the 

 reproduction following fertilization which he calls "propogative 

 reproduction." A relativeh' simple, but very interesting life cycle 

 is described by Leger in the case of Ophryocystis mesnili found in the 

 Malpighian tubules of the beetle Tenebrio vioUtor (Fig. 180). Here 

 the asexual cycle is reduced to a process of equal division or multiple 

 division whereby a number of gamonts are formed. These gamonts 

 unite two by two in pseudoconjugation. The nucleus of each 

 divides twice and one only of the resultant four nuclei becomes 

 the nucleus of a gamete. The two gametes become freed in a brood 

 chamber where they unite and in which the z.^gote gives rise to a 

 single sporoblast with eight sporozoites. 



In Schkocystis sipunculi and in Eleidheroschizon duhosqui the 

 asexual cycle is represented by a process of multiple unequal division, 

 the agametes being formed by a process of internal budding (Fig. 

 181). 



In some cases, particularly in the cephalont gregarines, special- 

 ized sporoblast disseminating tubes known as sporoducts, are formed 

 by the sporocysts. These are developed as ingrowths from the 

 cortical protoplasm which in the ripe sporocyst and under the influ- 

 ence of moisture are evaginated as tubular processes through which 



