222 ROBERT PAYNE BIGELOW ON 



These observations, then, confirm those of Glaus ('83,) who, without going into the 

 details of development, maintained that the rhopalia are modified basal portions of the 

 tentacles of the scyphistoma; and they contradict Goette's statement that the rhopalia 

 are developed independently of the tentacles. 



Other Phenomena of Stabilization. While the marginal structures are. undergoing 

 the metamorphosis that has just been described, important alterations are taking place in 

 the general shape of the body. The horizontal constriction first noticed in Fig. 21 has 

 deepened (Fig. 24), while the fold below it has heightened, and the upper portion has broad- 

 ened and flattened, until the condition shown in Fig. 26 is reached. At this final stage 

 the upper portion has all the characteristics of a free-swimming medusa (ephyrula), except 

 that it is attached by a slender aboral stem to the centre of a goblet-shaped basal polyp. 

 The four interradial depressions in the peristome of the earlier stages have become nearly 

 flattened out, all that remains of them being the hollows between the projecting radial 

 angles, or pillars, of the proboscis, which have now become very prominent. 



At the stage of Fig. 21, there may be noticed on the proboscis eight patches of 

 thickened ectoderm containing many nettle cells. These nettle batteries are arranged 

 symmetrically, one on each side of each pillar of the proboscis. At a little later stage 

 (Fig. 24) the batteries have become invaginated, forming cup-shaped depressions, thickly 

 crowded with nettle cells in all stages of development. 



During the time when the larva is being differentiated externally into an upper and 

 a lower portion, internal changes are taking place in the former, which result in the 

 disappearance of structures characteristic of the scyphistoma and the appearance of 

 others distinctive of the medusa. 



The orifices in the gastric septa have become relatively larger (cs. Fig. 52 and re. 

 Fig. 57) until the septa are reduced to columnar pillars, columellae, connecting the upper 

 and lower walls of the body and pierced longitudinally by the septal muscles (c. Fig. 

 58) . The columellae are called by German authors "Septalknoten," but they are not 

 homologous with the so-called " Septalknoten," or areas of adhesion, in the Peromedusae. 

 The columellae of the Peromedusae are the walls of the large septal funnels where they 

 pass from the subumbrella to the exumbrella, and, according to Haeckel's figures, are 

 separated by the gonads from the areas of adhesion. 



In the fully developed scyphistoma of Cassiopea, the septal muscles are solid 

 throughout their length, and there is no cavity corresponding to the septal funnels, 

 which, according to Goette, are well developed in Aurelia. But in the strobila there 

 does appear a slight depression extending a very short distance into the end of each 

 septal muscle. In Figs. 57 and 58, where this is well marked, much of the depression 

 may be due to the strongly contracted condition of the specimens; but other specimens 



