[15] MEDUSA FROM THE GULF STREAM. 941 



Habitat. — This species has been collected, as shown in the above table, 

 between latitudes 38° 19' 26" north and 40° 4' 20" north ; longitudes 

 G80 10' 00" west and 71° 4' 00" west. It was taken from 373 to 2,369 

 fathoms. 



Umhrella, — Two different regions of the umbrella, called the aboral or 

 exumbral and the oral or subumbral side, can be distinguished in these 

 specimens oiAtolla. If the medusa is free-swimming, the former is prob- 

 ably uppermost as it moves in the water, but the stiff, gelatinous, al- 

 most cartilaginous character of the disk suggests that it has little power 

 of motion in the central disk.* The umbrella is flat, discoidal, of a slight 

 blueish tinge. The consistency of the bell is at times cartilaginous. The 

 wall of the exumbrellais divided by a circular furrow {fos. cor.), which is 

 almost as deep as the thickness of the umbrella. In two specimens well- 

 marked radial lines or slight depressions, sulci radiales (s. r.), are found 

 on the peripheral region of the central disk; in the other specimens 

 these markings are not as plainly seen. When present the number of 

 radial markings {s. r.) on the exumbral surface of the central disk corre- 

 spond with the number of tentacles or marginal sense-clubs. These 

 bodies correspond with what Hgeckel has described in WyviUii as tlic 

 " sulci radiales." In A. WyviUii, however, they are broader as compared 

 with their length than in this species. The corona of the umbrella is 

 seen on the exumbral side to be marked off into two zones of gelatinous 

 blocks, the innermost zone [soc. ta.) bearing the tentacles ; the outer- 

 most, the sense-bodies. In some of the specimens the exumbral sur- 

 face of the corona appears almost smooth, in others the outer sur- 

 faces of the gelatinous blocks are rough or channeled. The external 

 faces of the tentacular ring of blocks are more prominent than those 

 of the sense-organs, and are well marked on larger specimens. There 

 are as many of the tentacular blocks as of tentacles. The inner faces 

 of the tentacular blocks are fastened to a flat circular muscle, which 

 forms the outer wall of the coronal ditch. On each side the tentac- 

 ular block fits closely upon its neighbor and fuses with it, foruiing 

 a continuous ring. On the exumbral side the block is slightly con- 

 vex, and on either side, extending from the upper surface to the sides 

 which adjoin, is a small polygonal facet. The division of the tentacular 

 blocks is superficial. The base of the tentacle occupies an abaxial facet, 

 while the two lateral faces leave an interval into which fit the alter- 

 nating gelatinous bodies of the sense-blocks. The tentacles are in most 

 of the specimens short, stiff, and very cartilaginous. Although Hteckel 

 describes their base as penetrated by a tube in his species, in Vcrrillii 

 I was unable to discover any cavity or ccecal ending of a tube. On the 

 side turned to the central disk there is a strong muscle, which arises from 



* The great development of the subumbral coroual muscle indicates that Atolla does 

 not resemble Cassiopea in living upon the bottom. It is probably a free-swimming 

 mednsa. A proboscis, like that of Atolla, implies food of large size, and not the small 

 animals upon which Cassiopea feeds. 



