DERIVATION OF MEDUSA FROM HYDRANTH 239 



A', transverse section of the same through the plane (a b}. 



B, the tentacular region is extended into a hollow disc. 



c, the tentacular region has been further extended and bent into a 

 bell-like form, the enteric cavity being continued into the bell (ent. cav 1 ) : 

 the hypostome now forms a manubrium (mnb}. 



c', transverse section of the same through the plane a b, showing the 

 continuous cavity (ent. cav 1 } in the bell. 



D, fully formed medusa : the cavity in the bell is reduced to the 

 radiating (rad. c) and circular (cir. c] canals, the velum (v) is formed, 

 and a double nerve-ring (nv, nv 1 ) is produced from the ectoderm. 



D', transverse section of the same through the plane a b, showing the 

 four radiating canals (rad. c] united by the endoderm-lamella (end. lam), 

 produced by partial obliteration of the continuous cavity ent. cav 1 in c 1 . 



outer surfaces by a thin layer of delicate cells (ect). The 

 clapper-like organ or manubrium (Fig. 54, c and Fig. 55 D 

 and D', mnb') is formed of two layers of cells, precisely 

 resembling the ectoderm and endoderm of Hydra, and 

 separated by a thin mesoglcea ; it is hollow, its cavity (Fig. 55, 

 D, ent. cav) opening below, i.e. at its distal or free end, by 

 a rounded aperture the mouth (mt/i), used by the medusa for 

 the ingestion of food. At its upper (attached or proximal) 

 end the cavity of the manubrium is continued into four 

 narrow, radial canals (Fig. 54, c, rad. c', and Fig. 55, D and 

 D' rad) which extend though the gelatinous substance of the 

 bell at equal distances from one another, like four meridians, 

 and finally open into a circular canal (cir. c) which runs 

 round the edge of the bell. The whole system of canals is 

 lined by a layer of cells (Fig. 55, D and D', end) continuous 

 with the inner layer or endoderm of the manubrium ; and 

 extending from one canal to another in the gelatinous 

 substance of the bell, is a delicate sheet of cells, the endoderm- 

 lamella (D', end. lam). 



From the edge of the bell four pairs of tentacles (Fig. 54, 

 c and Fig. 55, D, /) are given off, one pair corresponding to 

 each radial canal, and close to the base of each tentacle is a 

 little speck of pigment (ot), the ocellus or eye-spot. Lastly, 



