162 



COELENTERATA. 



the disc and on the tentacles of many Medusae enables them to cause 

 a perceptible stinging sensation on contact. Many, e.g. Pelagia, are 

 phosphorescent. According to Panceri, this phenomenon originates in 

 the fat-like contents of certain epithelial cells on the surface. The 

 Acalephae may attain a large size, the bell in some Rhizostomae 

 and Cyaneidae reaching a diameter of from two to six feet. 



In spite of the delicacy of their tissues, certain large Medusae have 



left impressions in the litho- 



rl 



graphic slate of Sohlenhofen 

 (Medusites circularis, etc.). 



Order 1. 



SCYPHOMEDUSAE. 



TETRAMERALIA. TESSERONAE. 



Acalephae with or without 

 four rhopalia (sense-tentacles) ; 

 stomach with four gastral 

 pouches separated by short or 

 long septal-imions. Gonads in 

 sub-umbrella wall of the gas- 

 tral 2>ouches. Umbrella highly 

 vaulted. 



The Scyphomedusae are best 

 considered in their relation to 

 the Scyphistoma. They may 

 be looked upon as Scyphistomas 

 deprived of their tentacles, 

 which indeed are only transi- 

 tory structures, and elongated 

 so as to assume the form of a 

 cup, and changed in several par- 

 ticulars which are characteristic 

 The four septa (Fig. 98) arise by the fusion 

 of the four gastric folds with the wide oral disc, which becomes 

 drawn in and concave like a sub-umbrella. These four septa separate 

 the same number of gastrovascular pouches ; while the margin of 

 the cup may be drawn out into eight arm-like processes from which 

 groups of short, knobbed tentacles arise (Fig. 134). 



The genital organs extend on the oral Avail of the umbrella into 

 the arms as eight band-shaped, plicated ridges (Fig. 134, /.). They 

 run along in pairs at the lower part of each septum in the gastric 



FIG. F132. g, fully formed strobila with sepa- 

 rating ephyrae; h, free ephyra (1-5 to 2 mm. 

 in diameter). 



of the medusa stage. 



