HIGH .SCHOOL ZOOLOGY. 



223 



so-called water-vascular system — to be presently described — 

 replaces the segmental oi'gans. 



8. The Ptotifei-a or Wheel- Animalcules are microscopic aquatic creatures, 

 round the mouth of which are disposed lobes bearing cilia, which, wlieu 

 in motion give the apjjearance of rotating wheels (Fig. 150). They serve 



to bring food to the mouth, and also for 

 swimming. A longer or shorter tail, which 

 is sometimes telescopic, assists in locomo- 

 tion, and serves also, as it terminates in a 

 pair of forceps, for temporary fixation. Some 

 of the species are sedentai'y, living in tubes, 

 either singly or in colonies. The intestine is 

 absent in the males (which are not only more 

 minute than the females, but also much fewer 

 in number, and shorter-lived), but it is com- 

 plete in the females, having near the mouth 

 an expanded part containing a chitinous mas- 

 ticating apparatus, and, behind that, two 

 lateral cceca. The water- vascular system 

 consists of two convoluted tubes opening 

 anteriorly into the ccelom and posteriorly 

 into a contractile bladder, which communi- 

 cates with the rectum. Instead of the elong- 

 ated nerve-cord of the higher Annelids, there 

 is here only a single ganglion situated above 

 the oesophagus, whence are distributed nerve- 

 fibres to the various parts, including the eyes 

 and tactile organs. There are no blood- 

 vessels, and no respiratory organs. The Rotifers hke the Tardigrades 

 have considerable power of resisting death by desiccation, 



9. The names of the remaining Classes, which include for the 

 most part parasitic worms, are taken from the form of the body, 

 which is cylindrical in the Nematelminthes but flattened in the 

 Plathelminthes. According to the grade of parasitism, the in- 

 testinal system is more or less reduced, being absent in those 

 most completely adapted for a parasitic mode of life. Organs of 

 fixation suited to the nature and degree of the parasitism, are 



Fig. 150.— Female of Roti- 

 fer. {Ilydatina senta). 



