i6 4 



GENERAL BIOLOGY 



The ventral surface of the worm is somewhat flattened, 

 and at either side of it are two double rows of minute setae 

 with their tips protruding through the skin. They are 

 short and stiff, and not at all conspicuous indeed, are 

 easier felt than seen, as one may demonstrate by drawing a 

 worm backward between the thumb and finger. It is by 

 means of these that the worm maintains its footing in 

 crawling, or its hold within its burrow. 



The only conspicuous external feature is the broad girdle 

 or'clitellum that surrounds the body between the thirtieth 

 and fortieth segments. Although inconspicuous,the openings 

 of the ducts from the reproductive organs may be seen at the 

 sides of the ventral surface of segments fourteen and fifteen, 

 between the rows of setae. 



Internal Features. The body of the hydra is tubular in 



plan; that of the 

 worm is compound- 

 tubular a tube with- 

 in a tube; the inner 

 tube is the food tube, 

 alimentary canal or 

 enteron; the outer is 

 the body wall; the 

 space between is the 

 body cavity or ccelom 

 (see fig. 103). 



If a worm be anes- 

 thetized with chloro- 

 form, and a slit be 

 made through the 

 body wall along the 



mid-dorsal line, and the cut edges be drawn apart for 

 inspection of the interior, it will at once be seen that the 

 segmentation of the worm extends to the internal organs 



FIG. 103. Diagrams of the structure of hydra and 

 worm, o, cross, and b, long-sections of hydra; 

 c and d, corresponding sections of the worm. 

 The ectoderm is stippled, the endoderm, solid 

 black and the mesoderm of the worm, cross- 

 lined; o, coelom. 



