114 REPORT OF THE COMMISSIONER OF FISH AND FISHERIES. 



a satisfactory classiticatioii of the order it will be necessary to utilize 

 additional characters, in particular those connected with spore topog- 

 raphy and spore symmetry. This brings us to a consideration of the 



SYMMETRY OF THE MYXOSPORIDIAN SPORE. 



Considering the importance of the presence or absence of symmetry 

 throughout the animal kingdom, it is strange that no attention has 

 heretofore been paid to this feature of the myxosporidiau spore. These 

 bodies exhibit four varieties of symmetry, viz: 



1. Absence or obscurity of symmetry. — Tliis is found in the Cryptocystes. 

 Anteroposterior symmetry is certainly absent; bilateral and supero- 

 inferior symmetry (or asymmetry) obscure. 



2. Bilateral symmetry (symmetry around the vertical plane). Present 

 in all genera of Fhwnocystes except (Jeratomyxa,^ which is asymmetric 

 as regards the position of the sporo])lasm. 



3. Supero-inferior symmetry (dorso- ventral symmetry; symmetry 

 around the longitudinal plane). — This is the rule in the Pluenocystes, 

 but as no attention has been directed to the detection of asymmetry, 

 it may be that it is present in a few species. It certainly forms a 

 striking feature of Myxobolns macrurns, in which the dift'erentiation of a 

 dorso-ventral axis is perfectly plain. Further, the supero-median cornu 

 extends farther forward than the inferior median cornu in several (all 

 examined by me) Myxoholus species, furnishing another indication of 

 this differentiation and a clue to the homology of the superior and 

 inferior surfaces in different spores (see pp. 122, 235). 



4. Antero-posterior symmetry (symmetry around the tracsverse plane). 

 This type appears to be characteristic of, and confined to, the genus 

 Cystodiscus, in which antero-posterior symmetry is equally present, 

 whether we regard the extremities of the spores as (anterior and pos- 

 terior) ends or as (right and left) wings. 



The importance, for classification, of a study of spore symmetry is 

 soon seen. Employing the knowledge thus obtained for the purpose of 

 orienting the spore, we find that the characters of greatest taxonomic 

 value are : 



1. Spore topography. — Thus in Myxidium lieberhlihnii the presence of 

 bilateral and the absence of antero-posterior symmetry show that the 

 two pointed extremities of this spore, heretofore, like all other pointed 

 extremities, loosely termed " ends," do not correspond to anterior and 

 posterior, but to right and left. On the other hand the "ends "in 

 Cystodiscus appear to represent ends sens, strict., i. e., to correspond 

 to anterior and posterior. 



1 With the further exception of two Myxoholus species (M. unwapsulatus with only 1 

 capsule, and M. inequalis with 2 unequal capsules), which, on account of reduction 

 of characters, have suft'ered a corresponding loss of the perfect symmetry character- 

 istic of the genus. To make the exception absolutely complete, M. stronyylurua may 

 be added (see p. 249). 



