THE EARTHWORM 



to the thirty-fifth segments, known as 

 the clitellum (Lat. clitellce = saddle) ; 

 Fig. 74. These segments are larger than 

 elsewhere and have a thicker wall and 

 special functions. At the extreme pos- 

 terior end the segments become smaller, 

 and the last one has an opening which 

 is the posterior opening of the diges- 

 tive tract, the vent or anus, a. Be- 

 cause of this ringed structure the 

 earthworm belongs to a class of animals 

 called Annulata (Gr. annulus = ring). 

 Structure of the Body. The body 

 of the earthworm can be compared to 

 a tube within a tube; Fig. 76. The 

 outer tube is called the body wall, 6, 

 and the inner tube the alimentary 

 canal or the digestive tract. Between 

 the body wall and the digestive sys- 

 tem is a space filled with a liquid, this 

 space being a true body cavity or ccelom 

 (Gr. koilos = hollow), c, differing thus 

 from Hydra, that has no ccelom. The 

 body cavity is not, however, an open 

 space extending from the anterior to 

 the posterior end, but is divided by 

 partitions into a series of chambers, 

 with a chamber for each segment. The 

 partitions are called septa (sometimes 

 called dissepiments) . There are minute 

 openings through each septum, so that 

 the liquid that fills the body cavity 

 may pass through; thus the different 

 chambers are in communication with 

 each other. 



FIG. 76. DIAGRAM SHOW- 

 ING THE ANTERIOR END 

 OF THE EARTHWORM CUT 

 LENGTHWISE THROUGH A 

 VERTICAL MEDIAN LINE 



6, body wall; od, oviduct; 



c, coslom; 

 cr, crop ; 

 g, gizzard; 

 m, mouth; 

 mu, muscles; 

 n, nephridia; 

 o, ovary; 



oe, oesophagus; 

 ph, pharynx; 

 s, septa; 

 ap, spermary; 

 sr, seminal recep- 

 tacles; 

 t, typhlosole. 



