48 ILLINOIS BIOLOGICAL MONOGRAPHS [286 



The surface of the shell may be smooth or exhibit various markings. 

 More or less conspicuous ridges varying in form and number in different 

 species, may run parallel to the sutural line, may show a network-like 

 structure or may exhibit short tooth-like processes arising from the sutural 

 ridge and radiating toward the center of each valve. When the ridges are 

 fine, they form delicate striations, arranged usually parallel to the sutural 

 line. Tho these markings are usually easily seen in vivo, they are very 

 often more readily studied in stained preparations. 



Inside of the shell are present the polar capsules and sporoplasm. 

 Gurley (1894:120) and Davis (1917:210) used the term "capsules" instead 

 of polar capsules because of the facts that "the situation implied by the 

 the latter (polar capsule) is not constant" (Gurley) and that "they are 

 often not in the position indicated by the term polar capsule" (Davis). 

 The present writer, however, does not agree with these authors and retains 

 the commonly used term, polar capsule, thruout the present paper on 

 the basis of the fact that these polar capsules are situated at or near the 

 more or less attenuated anterior end in the great majority of species or at 

 each end (in Myxidiidae) of the spore, except in the few cases as in Wardia 

 in which they are situated in the central portion and have the foramina at 

 the anterior end of the spore. 



The polar capsules may be pyriform or spherical. They are located 

 at or near one end (anterior end) of the spore. In Myxidiidae, one polar 

 capsule is situated at each end, in which case no distinction can be made 

 between the anterior and posterior ends. The end or side opposite to the 

 anterior, is the posterior end of the spore. The number of polar capsules 

 in a spore varies according to the dififerent genera. There is only one polar 

 capsule in the spore of unicapsular Myxobolus, four in Chloromyxum, two 

 in all the other genera. They may be equal or unequal in form and size. 

 When two polar capsules are located at the anterior end, they may be 

 convergent or divergent. Each has a foramen to the outside of the spore 

 thru the shell in or near the sutural line, thru which the polar filament is 

 extruded. The foramen is observable in the fresh condition. Staining 

 will very often show clearly the canals thru the shell. Each polar capsule 

 has an independent foramen. 



In the polar capsule exists a coiled polar filament, which in most 

 cases can be recognized without diflficulty in the fresh condition. The 

 polar filament is as a rule a more or less extended, probably hollow thread 

 connected with the polar capsule, which is extruded from the spore thru 

 the foramen under the action of the stimulants such as the digestive fluid of 

 the host or certain chemicals. In Sphaeromyxa it is rather short and thick, 

 tapering to a point. The polar filament is coiled around the longest 

 axis of the polar capsule, except in Sphaeromyxa in which it is coiled 

 around an axis perpendicular to the longest axis of the polar capsule. 



