186 PROTOZOA 



progamic development (schizogony) leads as a rule to autoinfection, to increase of 

 the parasite in the tissues of the host. The parasite increases in size and breaks 

 up into numerous young (merozoites), which grow in turn and divide. This 

 process may continue many times until a sexually ripe form appears. Only in 

 the gregarines is this replaced by a single large growth, which, in the period of 

 preparation for fertilization, is divided into many parts. Fertilization usually 

 precedes encystment, only rarely occurring in the cyst. Occasionally there is a 

 fusion of isogametes; usually there are non-motile' macrogametes fertilized by 

 extremely active microgametes. In this the sexual dimorphism may be pre- 

 pared long before and only be expressed in the generation (gametocytes) from 

 which the macro- and microgametes arise. The metagamic development 

 (sporogony) requires encystment and serves to introduce the germs into a new- 

 host. Inside of the cyst spores are formed, which are rarely naked, usually 

 surrounded with a firm envelope. In the spore the sporozoite arises, the starting 

 point for the progamic development. In all of the divisions which precede 

 fertilization or immediately follow it, a part of the protoplasm, containing degen- 

 erating nuclei, remains behind as the residual body. From the development 

 thus outlined the Myxosporida and Sarcosporida differ in some points, though 

 they form spores .and sporozoites for new infections. 



Order I. Gregarina. 



The typical and longest known sporozoa are the Gregarines, parasites 

 of oval or thread-like form (recalling round worms), usually somewhat 

 flattened, which have only been found in the intestine or gonads, more 

 rarely in the body cavity, of invertebrates. The protoplasm (fig. 145, A ) is 

 separated sharply into a clear ectosarc (ek) and a granular entosarc (en). 

 The ectosarc is covered by a cuticle (/), permeable by fluid food, for no 

 cytostome exists. In many (perhaps all) there is a double striping of the 

 body, a longitudinal recognizable by furrows on the surface and hence 

 cuticular, and a transverse marking in the ectosarc, produced by circular 

 or spiral muscle fibrilke. These muscles explain the peristaltic motion 

 and the occasional bending of the body, but not the peculiar gliding 

 motion by which locomotion is usually effected. It may be that the greg- 

 arines secrete stiff gelatinous threads from the posterior end, and the 

 elongation of these forces the body forward. 



In many gregarines (Polycystidae) the body is divided into a smaller anterior 

 part^ the protomerite, and a larger deutomerite (fig. 145, A). Internally this 

 division is marked by a bridge of ectosarc across the entosarc. The vesicular 

 nucleus (there is but one in any gregarine) lies in the deutomerite. All gregarines 

 are parasitic in youth wholly inside of cells or with the anterior end imbedded 

 in the^host cell, which they leave in the developed stage. Many remain for a 

 long time with a process of the protomerite in the cells. This process the 

 epimerite is provided with threads or hooks for anchorage, and is usually lost 

 when the animal gives up its connection with the host cell. Among the intestinal 

 gregarines frequently occur 'associations' where two or more animals are fast- 

 ened together head to tail in a row (fig. 145, A). Perhaps these associations 

 are preparations for conjugation which occurs in development. 



