548 BIOLOGY OF THE PROTOZOA 



myxida) or with a single valve (Microsporidia) and contain one or 

 more polar capsules which recall the stinging cells of the Coelenterata. 

 The threads of the capsules are probably hollow and are spirally 

 wound in the capsule from which they are evaginated under proper 

 conditions. Such threads, the function of which is entirely prob- 

 lematical, may be short or very long, reaching in some cases a length 

 many times that of the sporocyst. The germs can scarcely be called 

 sporozoites since they are not formed as a result of metagamic 

 divisions following fertilization. The term sporoplasm has been 

 used to distinguish the vital, living portion of the spore from the 

 other differentiated parts and will be used here to designate the 

 young germ up to the time of development into the trophic indi- 

 viduals. The spores are built on the same general plan of structure 

 (Fig. 219). 



The form assumed by the trophozoites varies with the habitat. 

 Many of the Cnidosporidia are lumen-dwelling, and many are cell- 

 dwelling or tissue parasites. The free forms are characterized by 

 relatively complex organization with ectoplasm, endoplasm and 

 pseudopodia similar to amebae. The pseudopodia may be filiform, 

 lobose or lamellate and locomotion is frequently as active by ame- 

 boid movement as in many amebae. Tissue- or cell-dwelling forms 

 are active only in the young stages and according to Doflein may 

 appear in the following conditions: (1) Enclosed in cysts which are 

 formed for the most part by concentric layers of connective tissue 

 derived from the host, and an innermost layer formed by the 

 organism. Huge cysts resulting from association of parasites, and 

 easily visible to the naked eye, are formed in many cases. (2) 

 " Diffuse infiltration," a term used to indicate collections of parasites 

 between tissue cells where they may fill up cavities without doing 

 much or any harm to the host. (3) Intracellular parasites whereby 

 the usually minute organisms live at the expense of the cell host. 



Order 1. Myxosporidia BCtschli. 



The Myxosporida are the best known of the Neosporidia both 

 as to number of species and life histories. Of the 249 species listed 

 by Kudo (1919) all but 11 are parasitic in fishes, 5 have been found 

 in amphibia, 4 in reptiles, 1 in an insect and 1 in an annelid. They 

 are, therefore, characteristic fish parasites, where they occur both 

 as celozoic and as histozoic forms, never, according to Davis (1917), 

 in the digestive tract, but the free forms mainly in the gall and 

 urinary bladders, the tissue parasites mainly in the connective and 

 muscular tissues. The free forms produce no evident harmful effects 

 on the host but the tissue parasites are more disastrous, Myxobolus 

 pfeifferi, for example, causing costly epidemics amongst food fishes, 

 particularly in the barbel (Barbus barbus L.) of Europe. 



