^8o FRESH-WATER BIOLOGY 



weather approaches there is a return to the stouter forms. There 

 results an immense number of different forms, many of which have 

 been described as different species. Apparently these changes are 

 adaptive in character. At the higher temperatures of summer the 

 inner friction of the water decreases much (as Ostwald has shown) , 

 so that swimming animals tend to sink more readily than be- 

 fore. The decrease in size of the body, with the roughening of its 

 surface, increases greatly the proportion of body surface to body 

 weight, so that the animals sink less readily; the tendency to sink 

 due to the warmth of the water is compensated. The small, rough 

 forms are therefore adapted to warm ,weather. But the decrease 

 in size of the spines cannot be accounted for in this way; it must 

 depend on other relations. 



In the Rotifera the males are usually minute, degenerate crea- 

 tures, — the race being represented mainly by the females ! The 

 males usually have no alimentary canal, and thus during their 

 entire life they never take food. They are, of course, therefore, 

 condemned to an early death. They usually swim about rapidly, 

 often swarming about the females. Fecundation takes place in 

 some cases by the insertion of the copulatory organ of the male 

 into the cloaca of the female; this has been seen in many cases. 

 In other cases apparently the male pierces the body wall of the 

 female, injecting the spermatozoa directly into the body cavity. 

 This takes place in Hydatina. 



In a few of the Rotifera the males are of the same size and struc- 

 ture as the females (in the Seisonacea). In Proales werneckii the 

 male is of the same size and form as the female, but the aHmentary 

 canal, while present, is simplified and reduced in size. In other 

 species, various vestiges of the alimentary canal may be present, 

 but they are not functional. In certain groups no male is known 

 to exist; this is true for the entire suborder of the Bdelloida. In 

 the Rattulidae likewise no males have as yet been seen. Much 

 further study of the existence, structure, and activities of the males 

 is needed. If they are actually non-existent in some groups, then 

 of course the reproduction is throughout by parthenogenesis, — 

 fertilization of the egg not occurring even at long intervals. 

 Most rotifers produce several different sorts of eggs. These are 



