212 BULLETIN OF THE BUREAU OF FISHERIES. 



For descriptive purposes it is necessary to recognize certain topographic features 

 in the spore. Unfortunately, somewhat ambiguous terms have been used by various 

 writers which are likely to lead to confusion. The most objectionable are the terms 

 "anterior" and "posterior" ends, first used in this connection by Thelohan. While 

 the terms may not be open to serious objection in the case of Myxobolus, they are very 

 inappropriate, to say the least, when applied to the spores of certain other genera. 

 On this account it has seemed best to propose a slightly different terminology than that 

 used by previous writers. 



The position of the capsules in such genera as Ceratomyxa, Leptotheca, or Myxobolus 

 indicate the capsular side or end (anterior end of Thelohan). The opposite side sur- 

 rounding the sporoplasm is designated the postcapsular side. The plane extending between 

 the capsules from the capsular to the postcapsular sides and dividing the spore into two 

 symmetrical halves is the transverse plane (vertical plane of Gurley). The transverse 

 plane is the plane of symmetry and may or may not coincide with the sutural plane. 

 The longitudinal plane is the plane passing through the capsules and sporoplasm at 

 right angles to the transverse plane and divides each valve into a superior and inferior 

 half which are symmetrical with each other. The vertical plane (transverse plane of 

 Gurley) is the plane passing approximately between the capsules and the sporoplasm 

 at right angles to both the transverse and longitudinal planes, dividing the spore into 

 unsymmetrical capsular and postcapsular regions. 



The longitudinal axis (fig. 2, A-B) is the hne formed by the intersection of the 

 vertical and longitudinal planes. The transverse axis (fig. 2, C-D) is the Une formed 

 by the intersection of the transverse and longitudinal planes. The vertical axis (fig. 2, 

 E-F) is the line formed by the intersection of the vertical and transverse planes. 



The length of the spore is the distance along the longitudinal axis, and in case the 

 valves are curved the axis is supposed to have a corresponding curvature. The width 

 is the distance between the capsular and postcapsular sides along the transverse axis, 

 while the thickness is the length of the vertical axis. In the case of the capsules when 

 only one dimension is given it is always the longest diameter. 



Although always constructed on the same fundamental plan, the modifications of 

 form and structure which the spores may exhibit in different species are almost endless. 

 However, even a cursory study shows that some modifications are much more funda- 

 mental than others. The former are of great value in differentiating between the 

 different f amiUes and genera, while the latter are our main reliance in distinguishing the 

 different species. Among the more important and deep-seated modifications may be 

 mentioned the general form of the spore and the position and number of the capsules 

 (of. fig. 3-4). The principal specific differences consist of minor modifications of form, 

 appendages of various kinds, markings on the exterior of the sporocyst, position of the 

 capsular pores, and size, including in the latter term not only the size of the spore as a 

 whole but the relative sizes of the different parts. 



Of course, as in all systematic work, there is the possibility of variations in these 

 characters within the Umits of the species. This is a problem regarding which there are 

 very Uttle data at present. There seems to be a general consensus of opinion among 

 writers on the Myxosporidia that the spores show comparatively little variation, but so 

 far as known there have been no special investigations in regard to this point. With 

 this belief in the constancy of the spores the writer is in general agreement. The greatest 



