136 



ZOOLOC4Y 



SECT. 



from its dilated base sends off four delicate tubes, the radial 

 canals (rad. c), which pass at equal distances from each other 

 through the substance of the umbrella to its margin, where they all 

 open into a circular canal (circ. c), running parallel with and close 

 to the margin. By means of this system of canals the food, taken 

 in at the mouth and digested in the manubrium, is distributed to 

 the entire medusa. 



The edge of the umbrella is produced into a very narrow fold or 

 shelf, the velum (Fig. 101, vl), and gives off the tentacles (t), which 

 are sixteen in number in the newly-born medusa (Fig. 96), very 

 numerous in the adult (Fig. 100). At the bases of eight of the 

 tentacles two in each quadrant are minute globular sacs (I), 

 each containing a calcareous particle or lithite. These are the 

 marginal sense-organs or lithocysts : they were formerly considered 



mnb 



FIG. 100. Obelia sp. A, mature medusa swimming with everted umbrella ; B, one-quarter 

 of the same, oral aspect ; circ.c. circular canal ; gon. gonad ; 1. lithocyst ; mnb. manu- 

 brium; mik. mouth; rad.c. radial canal; t. tentacle. (After Haeckel.) 



to be organs of hearing, and are hence frequently called otocysts : 

 in all probability their function is to guide the medusa by 

 enabling it to judge of the direction in which it is swimming. 

 The marginal organs, in this case, may therefore be looked upon 

 as organs of the sense of direction. 



The manubrium (Fig. 101, mnb) of the medusa consists of 

 precisely the same layers as that of the hydranth ectoderm, 

 mesogloea, and endoderm. The ectoderm is continued on to the 

 sub-umbrella, and then round the margin of the bell on to the 

 ex-umbrella, so that both surfaces of the bell are covered with 

 ectoderm. The endoderm is continued from the base of the enteric 

 cavity into the radial canals and thence to the circular canal, 

 so that the whole canal-system is lined by endoderm. In the 

 portions of the bell between the radial canals there is found, 



