160 AN INTRODUCTION TO ZOOLOGY 



Excretory organs and a complicated reproductive system make 

 their appearance in this phylum. The muscle fibers of Hydra 

 are simple specializations of cells that perform other functions; 

 but in Planaria, cells are set aside for no other purpose than to 

 give a high degree of contractility to the body. These cells 

 form distinct bands of circular, longitudinal, and oblique muscles. 

 Table V contrasts the structures of Hydra and Planaria. In 

 this way a clear idea of the advance in complexity exhibited 

 by the latter may be gained. 



3. ROUND WORMS ASCARIS 

 (Ascaris lumbricoides Linn.) 



External Features. Ascaris is a round worm parasitic in the 

 intestines of pigs, horses, and man. The sexes are separa.;, 

 The female, being the larger, measures from five to eleven inches 

 in length and about one fourth of an inch in diameter. The body 

 is light brown in color; it has a dorsal and a ventral white narrow 

 stripe running its entire length and a broader lateral line is present 

 on either side. The anterior end possesses a mouth opening, 

 surrounded by one dorsal and two ventral lips. Near the pos- 

 terior end is the anal opening, from which, in the male, extends 

 penial seta of use during copulation. The male can be distin- 

 guished from the female by the presence of a bend in the posterior 

 part of the body. 



Internal Anatomy. If an animal is cut open along the dorsal 

 line (Fig. 81), it will be found to contain a straight alimentary 

 canal, and certain other organs, lying in a central cavity, the 

 ccdom, a cavity met with now for the first time. The alimentary 

 canal (2) is very simple, since the food is taken from material 

 already digested by the host whose intestine the worm inhabits. 

 It opens at the posterior end through the anus, which is not present 

 in members of the phyla already discussed. A muscular pharynx 

 (i) draws the fluids into the long non-muscular intestine (2)- 



