Protozoa and Rot if era at Havana, III. 343 



extended until it includes the minutest creature ; for it is 

 through these simple, minute organisms that we may arrive 

 at the relations which exist between the inorganic and the 

 organic. 



The Rot if era constitute one of these groups of minute 

 animals. It is a somewhat small group, consisting of about 

 six hundred known species, one' hundred and seventy of 

 which have been found in the United States. They range in 

 length from .05 to 2.5 mm. The body is usually elongate, 

 sac-like or more or less oval, and is commonly provided with 

 two circles of cilia on the corona or frontal border of the head, 

 which, as the name indicates, have the appearance of two 

 small wheels. These cilia are used in locomotion, and also 

 assist the animal in obtaining food, since by their rapid vibra- 

 tion a stream of water is directed towards the mouth or buc- 

 cal orifice, carrying with it particles of food material. The 

 word rotifer is, however, misleading, since in many cases the 

 circles of cilia or ciliary discs are so modified that they have 

 lost all resemblance to a wheel. Sometimes but a single disc 

 is present ; again, there may be merely a row of cilia around 

 the anterior border of the animal; and in rare cases the cilia 

 are even entirely w'anting. Some of the Rotifera have the 

 body covered with a very thin chitinous external cuticula, 

 while a great many have a hard, inflexible carapace or lorica. 

 A number of species inhabit tubes, which they either secrete 

 or build up from surrounding debris and pellets of excreta. 



Members of this group are readily recognized l»y the pres- 

 ence of the mastax, a more or less irregularly three-lobed 

 muscular organ, containing the teeth, jaws, or tropin, as 

 they are variously called. These are composed of chitin, are 

 hard and durable, and are true masticatory organs. Although 

 some of the rotifers have no cilia, all of them agree in having 

 a mastax and trophi, and as these structures are peculiar to 

 the Rotifera they afford a ready means of identification. 



The mastax is usually situated in the anterior part of the 

 body, just behind the buccal orifice or lips, and a short 

 oesophagus connects it with a large stomach lying in the pos- 

 terior dorsal part of the animal. Many rotifers have the 



