230 



GENERAL ZOOLOGY 



is well cooked. The Bureau of Animal Industry, United 

 States Department of Agriculture, has officials at all the 

 large packing institutions, who examine microscopically for 

 Trichina and tapeworm larvse some of the muscle of every 

 animal killed. 



Trichina belongs to the class Nemato'da (Gr. nema, thread ; 

 eidoB, form) and the phylum Nemathelmin'thes (Gr. nema, 

 thread ; helmins, worm). 



The Rotifer. Sometimes when we examine with the com- 

 pound microscope the contents of a small drop of stagnant 

 water, we find very small, conical- 

 shaped animals (Fig. 114), with two 

 spines at the end of a short tail, 

 "looping" across the field of vision 

 with considerable vigor. Occasionally 

 they cease the inch-worm method of 

 getting along, and whirl away by 

 means of one or two circular rows of 

 cilia (Fig. 114, l) at the broad, ante- 

 rior end. These animals are called 

 rotifers. A common species is Brachi- 

 o'nus urceola'ris. In length they are 

 about .3 mm. (^ in.). Although very 

 small, they possess a complete diges- 

 tive system, with mouth, pharynx, 

 grinding-apparatus, stomach, diges- 

 tive gland, intestine and anus, an 

 excretory system (no circulatory 

 system), nervous .system, sense-organs, 

 and reproductive system. The indi- 

 viduals are male or female, but the 

 male is to be found only in the fall of the year, and even 

 then rarely. It is about one fourth as large as the female. 

 In some species of rotifers the male is unknown. 



FIG. 114. Rotifer. Much 

 enlarged. (After Weber) 



1, cilia ; 2, nerve-ganglion ; 

 3, eggs forming ; 4, eggs 

 formed; 5, excretory 

 organs, n e p h r i d i a ; 

 6, terminal spines 



