186 



the summer months. Thus in the vernal pulse of pala (type only) 

 reaching 716,982 on April 24, 1896, 19,056 individuals were para- 

 sitized by Bim&rium hyalinum Przesm., or by something very 

 similar to it, and 30,966 were infested by a fungus-like growth. This 

 is about 7 per cent, of the total individuals. Similar though less 

 pronounced outbreaks have - attended other vernal and autumnal 

 pulses. Species of Colacium are sometimes found attached to the 

 loricas of this s*pecies. 



Brachionus pala is exceedingly variable, especially in the matter 

 of the development of the posterior spines. Forms without the 

 spines (pala type) intergrade, by only slight gradations, into those 

 with fully developed spines (var. amphiceros). The angle which 

 these spines make with the lorica is also a matter of great variation, 

 in preserved material at least. Individuals with the spines at right 

 angles to the antero-posterior axis are occasionally seen. The 

 species also varies in the matter of the dorsal- ventral curvature of 

 the antero-median spines (var. dorcas}. Individuals with such 

 curved antlers are sometimes provided with posterior spines (var. 

 dorcas form spinosus}. I have followed Weber ('98) in placing 

 B. amphiceros Ehrbg., B. dorcas Gosse, and its form spinosus Wierz. 

 as varieties of B. pala. They do not, however, all stand upon an 

 equal footing. B. amphiceros grades imperceptibly into B. pala, and 

 has the same seasonal distribution. B. dorcas and its form 

 spinosus intergrade with each other as do pala and amphiceros, and 

 they also exhibit some intergradations with B. pala; but they are 

 winter varieties, or at least belong to the colder season, as will 

 appear later. Their differentiation in this respect is thus more 

 striking than that of B. amphiceros, and makes it probable that we 

 have in dorcas a seasonal variety of B. pala. Zacharias ('98) has 

 reduced B. pala to a variety of B. amphiceros because in his opinion 

 the latter is the more widely distributed form in certain pond waters 

 which he examined. This is a criterion which presupposes a wide 

 knowledge of distribution and numbers, and, furthermore, a basis 

 which can not fail to add to the confusion already existing in this 

 genus, since it is hardly to be hoped that it will lead to the same 

 conclusion in the hands of different investigators in different regions, 

 or even in different seasons and years in the same region. As an 

 illustration of the difficulties which might arise I may cite the yearly 

 averages of amphiceros and pala in the table on page 182. In three 



