160 



COELENTERATA. 



opposite tentacles first make their appearance ; not, however, simul- 

 taneously, the one appearing after the other, so that the young larva 

 about to develop into the Scyphistoma presents a bilaterally sym- 

 metrical structure. Subsequently the second pair appears 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 succession, the third and fourth pairs appear; and 

 soon after, in the plane of these latter, four longitudinal folds 

 (Fig. 132 d) of the gastric cavity are developed (radii of the second 

 order or of the gastric filaments and genital organs; often called 

 the interradii, see above, p. 118, note). 



d 



FIG. 132 a-li. Larval development of Chrysaora ; a, plauula with narrow enteron ; b, the same 

 after attachment and formation of mouth and commencing tentacles ; c, young scyphistoma 

 with 4 tentacles ; Csk periderm ; d, eight-armed scyphistoma with wide mouth ; M longi- 

 tudinal muscles of gastral ridges. 



The eight-armed Scyphistoma soon produces eight fresh tentacles 

 (Fig. 132 e\ which succeed one another in irregular succession, and 

 alternate with the tentacles already present. Their position deter- 

 mines the intermediate radii, or adradii, of the future 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 gemmation and fission. 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 



