LIFE-CYCLE OF " CYSTOBIA " lEKEGULAEIS (mINOH.). 83 



(3) The gametes are, to all appearance, perfectly similar 

 (isogamous). Examples: Gregariua, Lankesteria. 



lu all these cases there is permanent association between 

 ripe sporonts, followed bj common encystment. 



(4) The isogamous gametes (greatly reduced in number) 

 may develop parthenogenetically, the septum between the 

 two associates not breaking down. Example: Opliryocystis. 



(? 5) The gametes may develop parthenogenetically, the 

 association being only temporally (Selenidium ?). 



(b) Ccelomic forms. 



(1) The gametes are only very slightly differentiated, and 

 chiefly distinguishable by their nuclei. Permanent association 

 and common encystment are usual. Examples: Urospora, 

 Gonospora, and Monocystis. (This stage is easily deriv- 

 able from A 2.) 



(2) The gametes are quite isogamous. Precocious asso- 

 ciation occurs. Diplocystis, Diplodina (probably also 

 Cystobia). 



(? 3) The gametes may develop parthenogenetically, the 

 association being either permanent, in which case the septum 

 does not break down (Ceratospora ?), or else temporary, 

 the two associates afterwards sporulating separately (Gono- 

 spora varia ?). 



Detailed Summaey. 



My investigations have related chieflj' to Diplodina 

 (Cystobia) irregularis (Minch.), parasitic in Holothu- 

 ria forskali, and to D. (C.) minchinii Woodcock, from 

 Cucumaria pentactes and C. planci. All the material 

 was collected near Plymouth, in which neighbourhood these 

 Holothurians have a very localised distribution. The Cucu- 

 mariae are scarce, and the percentage of infected individuals 

 is very small. 



I have also examined the trophozoites of Diplocystis 

 schneideri, from a new host^ Periplaneta orientalis. 



