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MANUAL OF ELEMENTARY ZOOLOGY 



end.t. 



are set free by breaking away as the buds of Hydra are, 

 and carry out sexual reproduction as inde- 

 pendent individuals. These individuals differ 

 widely from the polyps, being, indeed, so unlike them that 

 their origin from the colony would never have been guessed 

 unless it had been seen to take place. They are small 

 jelly-fish or medusa.. Each has the shape of an umbrella 



with a short, thick 

 handle anda fringe 

 oftentaclesaround 

 the edge. The 

 convex upper side 

 is called the exum- 

 brella, the concave 

 lower side the 

 subumbrella, the 

 handle the manu- 

 brium. Around 

 the edge of the 

 umbrella a low 

 ridge projects in- 

 wards. This is 

 the velum and re- 

 presents a much 

 larger structure in 

 the same region 

 of many other 

 medusae. At the 

 Fig. no. — A longitudinal section of a hydranth end of the manu- 

 of Obelia, highly magnified. brium is the 



ect. t Ectoderm ; end., endoderm ; end.t. y endoderm of the mouth which 

 tentacles : hyth.. hydrotheca ; or.c. oral cone ; st.l., , , . ' , . . 



structureless lamella. leads by a tubular 



gullet along the 

 manubrium to a stomach in the middle of the body. 

 From this four radial canals run outwards to a ring 

 canal at the edge of the umbrella. The lining of all these 

 internal spaces consists of endoderm, and the radial canals 

 lie in a sheet of endoderm, known as the endoderm lamella. 

 In fact we may regard the internal cavities of a medusa 

 as corresponding to the enteron of a polyp in which the 

 walls have come together over a large area, leaving certain 



