SCYPHOZOA 177 



Scyphomedusae (but are present also in some Trachylina in the 

 Hydrozoa). They are minute tentacles which project at the end of the 

 interradial and perradial canals, which are continued into them. The 

 edge of the bell projects over them as a hood. In each apical endoderm 

 cell of the tentacle there is a crystal which according to some authors 

 is calcium oxalate. On one side of the tentacle is a pigment spot which 

 may be an ocellus, and near it are two pits lined with sensory epi- 

 thelium and said to be olfactory. In the neighbourhood of these 

 tentacles, then, all the senses appear to be localized. The tentaculo- 

 cyst (Fig. 133) is made up of two parts, a club-shaped projection 

 heavy at its distal end, and a pad of sensory epithelium immediately 

 beneath it. If the medusa is tilted from the normal horizontal position 

 the club of the highest tentaculocyst will press more firmly against its 

 sensory pad, and the club of the lowest tentaculocyst less firmly. 



Fig. 133. Diagram of tentaculocyst of Aurelia: A, in horizontal position; 

 B, with medusa tilted, the tentaculocyst t being pressed down upon the 

 sensory epithelium se. ; h, hood. 



Whatever tentaculocyst is highest produces greatest stimulation : this 

 alone controls the rate of beating of the bell, which has been shown to 

 be 50 % greater than normal when the animal is tilted through 90°. 

 Further the state of excitation of the highest tentaculocyst does not 

 allow complete relaxation of the musculature of the section of the 

 bell nearest to it between successive beats. This means that less water 

 is driven downwards at each beat from the uppermost half of the bell 

 than from the lower half, with the result that the bell automatically 

 rights itself. The Scyphomedusae are excellent subjects for experi- 

 ment, and if cut into ribbons will still live and their muscles function. 

 If the tentaculocysts are cut out one by one the rhythmic movements 

 of the bell continue until the last is removed when they suddenly 

 cease. After that, drastic stimulation, tactile or chemical, is necessary 

 to make the muscles contract. 



The gonads are situated, as has been already stated, in the floor of 

 the stomach, and the ripe gametes are liberated into the genital pouch. 



