THE WORM-LIKE ANIMALS 



351 



canal in a transverse section. The body-cavity of a frog is one con- 

 tinuous space, while that of mammals is divided by the diaphragm 

 into the thoracic cavity (containing heart 

 and lungs) and the abdominal with stom- 

 ach, intestine, liver, kidneys, and repro- 

 ductive organs. 



Internal Organs. (D) The teacher 

 should point out on preserved earth- 

 worms, which have been cut open along 

 the dorsal side and pinned out on a narrow 

 board, the following systems of organs : 



(1) Alimentary system consisting of: 

 pharynx (for seizing food) ; esophagus ; 

 crop (a reservoir for food) ; gizzard (for 

 grinding food) ; stomach-intestine (for 

 digesting and absorbing food. 



(2) Circulatory System. A large blood- 

 vessel on dorsal side of the intestine; a 

 number of arch-like branches which ex- 

 tend around the intestine to a ventral 

 blood-vessel. Through the skin of a small 

 living earthworm it is possible to see nu- 

 merous branches from these larger blood- 

 vessels to all parts of the body. In 

 addition to blood, the space (body-cavity) 

 between the digestive canal and the body- 

 wall contains a fluid which supplements 

 the work of the blood. Blood flows for- 

 ward in the dorsal vessel and backward in 

 the ventral. The dorsal vessel and the 

 arches have muscular walls which con- 

 tract at regular intervals, driving the 

 blood forward and then down into the 

 ventral vessel. In the posterior part of the 

 animal the blood gets back to the dorsal 

 vessel, thus making a complete circulation. 



(3) Reproductive Organs. These are 

 large, light colored organs lying on right 

 and left of the alimentary canal between 

 the tenth and fifteenth segments. The 

 animal is hermaphroditic ; that is, it has 



both ovaries and spermaries. Very small tubes, not easily seen, ex- 

 tend to openings on the ventral surface of the animal. 



FIG. 113. Diagram of in- 

 ternal organs of earth- 

 worm as seen dissected 

 from dorsal side, c.g, cere- 

 bral ganglion or " brain "; 

 ph, pharynx; ce, esopha- 

 gus; ao. 1, first of five 

 pairs of arches connecting 

 dorsal and ventral blood- 

 vessels; r, reproductive 

 organs ; c, crop in seg- 

 ment 15; g, gizzard; 

 s.i, stomach-intestine; 

 d.v, dorsal blood-vessel; 

 d, septum between two 

 segments. (From Sedg- 

 wick and Wilson.) 



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