256 CCELEXTERATA. 



formed as a perforation at the free end, the tentacles appear. As 

 in the embryo Actinia, two opposite tentacles first make their 

 appearance : not, however, simultaneously, the one appearing after 

 the other, so that the young larva about to develop into the Scyphis- 

 toma presents a bilaterally symmtrical structure. Subsequently 

 the second pair appear in a plane at right angles to the plane of 

 the first tentacles. These four tentacles mark the radii of the first 

 order. Then alternating with these, but in a less regular suc- 

 cession, the third and fourth pairs appear ; and soon after in the 

 plane of these latter four longitudinal folds of the gastric cavity 

 are developed (radii of the second order or of the gastric filaments 

 and genital organs). 



The eight-armed ticyphistoma soon produces eight fresh tentacles, 

 which succeed one another in irregular succession, and alternate with 

 the tentacles already present. Their position determines the inter- 

 mediate radii of the fiiture young Discophor or Ephyra, After the 

 formation of the circle of tentacles and the secretion of a clear basal 

 periderm (Chrysaora), the Scyphistoma is capable of reproduction 

 by fission and gemmation. At first the Scyphistoma appears to 

 multiply only by budding ; the second mode of reproduction, the 

 process of strobilization, begins later. This consists essentially in the 

 fission and division of the anterior half of the body into a number 

 of segments, thus changing the Scyphistoma to a Strobila. The 

 separation of the segments progresses continuously from the anterior 

 end to the base of the Strobila, so that after the disappearance of the 

 tentacles, first the terminal segment, then the second, and so forth, 

 attain independent existence. Each segment becomes an Ephyra, 

 developing eight pairs of elongated marginal lobes, with a marginal 

 body in the notch which separates the two lobes of the same 

 pair. It is these marginal lobes which give to the edge of the 

 umbrella of the Ephyra its characteristic appearance. The young 

 Ephyra gradually acquires the special peculiarities of form and 

 organization of the sexually mature animal (vide figs. 113 A). 



The number of nematocysts accumulated on the upper surface of 

 the disc and on the tentacles of many Medusas enable them to cause 

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

 phosphorescent. According to Panceri, this phenomena originates in 

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



In spite of the delicacy of their tissues, certain large Medusa 

 have left impressions in the lithographic slate of Sohlenhofen 

 (Mfldusites circularis, etc.) 



