ANNULATA (RING-WORMS) 141 



A fossil looking much like the living round-worm Sagitta has 

 been found in the Middle Cambrian shales of British Columbia. 



Derivation of name. — Greek nema, thread, + helmins, a 

 worm, referring to the round thread-like body. 



Living examples are A scar is lumbricoides (the round-worm 

 from the small intestine of man), Oxyuris vermicularis (the 

 pin-worm infesting the rectum of man), Trichina spiralis (para- 

 sitic in man and the pig) and Gordius (the hair-worm ; parasitic 

 in asexual generation, free in sexual ; in the latter it is often 

 15 cm. long by 5 mm. in diameter and popularly is supposed to 

 be derived from horsehairs which have fallen into the water). 



PHYLUM VI, TROCHELMINTHES (WHEEL-WORMS) 



All Trochelminthes, except the few belonging to the slightly 

 related orders Gastrotricha and Dinophilea, belong to the wheel- 

 worms (order Rotifera). These are microscopic in size, being 

 the smallest of all animals except the Protozoa. The anterior 

 end is surrounded by many variously arranged cilia, often in a 

 circle, by means of which the animal moves. Body unsegmented. 

 The oesophagus has a chitinous masticating apparatus. Anus 

 present. Sexes separate. Few species are parasitic. Unknown 

 in the fossil state. 



Derivation of name. — Greek trochos, wheel, + helmins, 

 worm, referring to the successive movement of the cilia at the 

 anterior end of the body which gives an appearance of rotation. 



Living examples are Branchionus (abundant in ponds and 

 ditches) and Rotifer. 



PHYLUM Vn, ANNULATA (RING-WORMS) 



All, except the class Gephyrea, have a ciliate, elongate body, 

 with mouth at one end and anus at the opposite ; the bilaterally 

 symmetrical body is divided by ring-like constrictions into a 

 series of segments representing a like segmentation within, where 

 each segment contains a portion of the digestive canal, a pair of 



