Notes on Medusa <f the Western Atlantic. 153 



white markings upon the surface were much fewer. These points of dif- 

 ference were so important that it was concluded that this specimen must 

 represent a different genus of Scyphomedusas. It was determined to be 

 Polyclonia frondosa. Much time was spent in trying to find other speci- 

 mens of the same form without avail. 1 The single individual found must 

 have grown up in the moat from a larva brought in accidentally by the tide. 

 The habit of these creatures is so excessively sedentary that it is incon- 

 ceivable that the adult creature could have been carried thither by ocean 

 currents. There may, of course, have been others in the same location, but 

 the most painstaking search failed to reveal them, and this in spite of the 

 fact that so large a part of the bottom is visible from a boat in clear weather. 



Oral scales. These noticeable flakes of white serve to distinguish the 

 species at a glance from the evenly yellow hue of Cassiopea. Their presence, 

 together with the absence of the digitate yellow appendages of Cassiopea, 

 is sufficient, to my mind, to separate the genera from one another. The 

 shape of these scales has been described. They are scattered over the arms, 

 between forty and forty-five on each arm. They serve as little lids, guard- 

 ing the openings into the oscula or oral funnels. They are sensitive to 

 touch, contracting and bending away from anything that touches them. 

 This reaction serves to bring the scale over the opening of the oral pores, 

 preventing the ingress of the disturbing object. There is nothing in Cassi- 

 opea which shows so great a degree of sensitiveness as these oral scales in 

 Polyclonia. 



Oral arms. Another point of difference was to be noted in the appear- 

 ance of the oral surface of the two forms of Rhizostome, viz, the relative 

 shortness of these processes in Polyclonia. In the individual examined the 

 arms were not visible projecting beyond the disk, as in Cassiopea. Their 

 length was a little less than the diameter of the disk. In Cassiopea, on the 

 other hand, the oral arms project beyond the margin. 



Surface of disk. There is a conspicuous band or ring of darker color 

 in Cassiopea, three-fourths of the distance from the center to the margin 

 of the smooth surface of the disk. It is just outside of a circle of large 

 oval white spots, which are more or less sharply separated from one another, 

 and it is bounded outwardly by a clean-cut band of whitish hue, which ex- 

 tends to the margin. This ring is slightly raised above the rest of the sur- 

 face, and when a jelly-fish is put into a glass jar it usually applies this part 

 to the surface of the glass, the disk inside the ring and the margin being 

 left free. The center of the disk is slightly concave, and acts, in this atti- 

 tude, as a cupping organ. If one tries to remove the creature from its 

 position, it will be found to require a vigorous pull to dislodge the disk from 

 its hold upon the glass. The cells of the dark ring secrete mucus, which 

 aids in giving a firm hold to the disk. 



1 Dr. Mayer reports the finding of six individuals of this species in the moat 

 during July, 1907. 



