THE NEOSPORIDIA 



399 



the sporozoites are always, apparently, amcebulse, and never 

 gregarinulse.* 



The Neosporidia are divisible into two sections, known re- 

 spectively as the Cnidosporidia and the Haplosporidia. The 

 Cnidosporidia are distinguished by the possession in the spore of 

 peculiar structures termed polar capsules, which are lacking in the 

 Haplosporidia. 



A polar capsule (Fig. 163) is a hollow, pear-shaped body, with a 

 tough envelope, probably chitinoid in nature. It is situated at one 

 pole of the spore, with its pointed end immediately below the 

 surface, in continuity with a minute pore in 

 the sporocyst. Coikd up within the capsule 

 is a delicate filament, often of great length, 

 probably of the same nature as the capsule, 

 and continuous with it. Under suitable stimu- 

 lation the polar filament is shot out through 

 the pore in the sporocyst. In their structure 

 the polar capsules resemble the nemetocysts 

 of the Ccelentera. Each polar capsule is 

 formed within a capulogenous cell. 



The Cnidosporidia comprise four orders 

 the Myxosporidia, Actinomyxidia, Micro- 

 sporidia, and Sarcosporidia. The Haplo- 

 sporidia constitute an order apart. 



Order I. : Myxosporidia. This order is 

 characterized chiefly by the following points : 

 The principal trophic phase is a multinucleate 

 plasmodium of relatively large size, resembling 

 an amoeba in its appearance and movements. 

 The spores are also relatively large, and 

 exhibit typically a binary symmetry, having 

 a sporocyst composed of two valves and 

 usually two polar capsules, sometimes increased 

 in number to four, rarely reduced to one. 



The Myxosporidia comprise a great number of genera and specie*?, 

 parasitic for the most part in cold-blooded vertebrates, especially 

 fishes, in which they are found very commonly. They are not as 

 yet known as parasites of birds or mammals, but a few species are 

 known from invertebrate hosts. 



Myxosporidia are typically tissue-parasites, occurring in various 

 tissues of the body, by preference muscular or connective, but also 



* A possible exception to this statement is furnished by the family Ccelospor- 

 idiidce of the Haplosporidia (p. 424). But the position of all the forms in this order 

 is more or less questionable, and their attachment to the typical Neosporidia is 

 still probationary. 



6 



FIG. 163. Polar cap- 

 sules of the spores 

 of Myxosporidia. a, 

 Polar capsule with 

 the filament coiled 

 within it ; b, with the 

 filament partly ex- 

 truded; c, d, with the 

 filament completely 

 extruded. After 

 Balbiani. 



