THE GREGARINIDA 



795 



cytes undergo karyokinetic division and the daughter nuclei pass to the 

 surfaces of the encysted parasites. The cytoplasm of each cell divides into 

 an equal number of segments and the segments collect around the nuclei : 

 each of these small nucleated masses of protoplasm is known as a sporoblast 

 or gamete. A certain amount of cytoplasm known as the cystal residuum 

 remains unused and serves for the nutrition of the gametes. The cuticle of 

 each gametocyte now dissolves ; the gametes exhibit active movement and 

 conjugate in pairs a gamete from one gametocyte conjugating notably 

 with a gamete from the other. After uniting each pair of gametes becomes 

 a zygote. The zygote becomes oval and secretes a chitinous envelope, forming 

 a sporocyst. Within this cyst the nucleus of the zygote or as it is now 

 termed the sporoplasm divides into two, then into four and finally into 

 eight nuclei which take up an equatorial position and become surrounded 

 each by a part of the protoplasm of the sporoplasm. In this way the sporo- 

 zo'ite or falciform body is formed. In Monocystis the fully formed sporozoi'te 

 has a more or less boat shape and resembles a diatom of the genus Navicella, 

 hence the name Pseudonavicella by which Gregarine spores are generally 

 known. 



[The spores of Monocystis do not appear to be able to develop further in 

 the earthworm but require to be transferred to a new host. It is probable 

 that infection of a new host takes place by way of the alimentary canal, the 

 digestive juices dissolving the wall of the sporocyst and setting free the sporo- 

 zo'ite which is actively motile and able to bore its way through cells and 

 tissues. 



[When Gregarines become encysted without pairing the gametocyte breaks 

 up into gametes at once. In these cases the spores are smaller than those 

 produced from zygotes.] 



FIG. 383. Gregarine of the lobster. On the right, the adult form of the 

 parasite ; on the left, the different shapes assumed by the protozoon in passing 

 from the young amo3boid to the adult form. (After V. Beneden.) 



From the observations of Caullery and Mesnil it appears that in some 

 Gregarines multiplication takes place by schizogony, the intra-cellular stage 

 of the life history being prolonged. In Gonospora longissima, a parasite of 

 Dodecaceria concharum, the sporozoite set free from the spbres in the gut of 

 the Annelid host passes into one of the epithelial cells lining the gut and in 

 this cell gives origin to a small spherical nucleated body which increasing in 

 size becomes the trophozoi'te ; and this multiplying by division divides into 

 six or eight crescent-shaped merozoi'tes arranged side by side to form a 



