iv PHYLUM C(ELENTERATA 135 



concave inner surface as the sub-umbrella. From the centre of 

 the sub-umbrella proceeds the manubrium (mnb), at the free end 

 of which is the four-sided mouth (mth). Very commonly, as the 

 medusa swims the umbrella becomes turned inside out, the sub- 

 umbrella then forming the convex surface and the manubrium 

 springing from its apex (Fig. 95, C, and Fig. 99, A). 



The mouth (Figs. 95, 96, 99, and 100, mth) leads into an enteric 

 cavity which occupies the whole interior of the manubrium, and 

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

 in mils {rod. 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 



ffon 

 mnb 



Kio. 99. — Obelia sp. A, mature medusa swimming with everted umbrella ; B, one quarter 

 of the same, oral aspect ; circ.c. circular canal ; gon. gonad ; I. lithocyst ; mnb. manubrium ; 

 mth. mouth ; rod. c. radial canal ; t. tentacle. (After Haeckel.) 



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. 100, vl), and gives off the tentacles (t), which 

 are sixteen in number in the newly-born medusa (Fig. 95), very 

 numerous in the adult (Fig. 99). At the bases of eight of the 

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

 each containing a calcareous particle or lithitc. These are the 

 marginal sense-organs or lithocysts : they were formerly considered 

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

 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. 100, mnb) of the medusa consists of 



