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 Rliizostomae 

 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- 

 graphic slate of Sohlenhofen 

 (Medusites circularis, etc.). 



Order 1. 



SCYPHOMEDUSAE. 



TETRAMERALIA. TESSERONAE. 



Acalephae with or without 

 four rliopalia (sense-tentacles) ; 

 stomach with four gastral 

 pouches separated by short or 

 long septal-unions. Gonads in 

 sub-umbrella wall of the gas- 

 tral pouches. 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 

 of the medusa stage. 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 wall 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). 



