344 MEDUS.E OF THE WORLD. 



The following are the dimensions ot a mature medusa which had 71 tentacles: 

 Diameter of bell, 19.5; height of bell, 9; length of tentacles, 15; width of velum, 3.5; 

 width of stomach at base, 4.5; length of stomach, 5; length ot each radial-canal, 10; length 

 occupied by gonad upon each radial-canal, 8. 



Adult medusa (plate 45, figs. I and 4; plate 46, figs. 1 and 2). — The bell is slightly flatter 

 than a hemisphere and is about 15 to 20 mm. in diameter. The gelatinous substance is quite 

 rigid. It is thin at the aboral pole of the bell and decreases regularly in thickness toward 

 the margin. At the base of each tentacle, on the velar side of the margin, there is a well- 

 developed oval bulb which is hollow, its lumen being connected with that of the ring-canal. 

 There are 60 to 80 stiff, slender tentacles which are about three-quarters as long as the bell- 

 diameter. The shafts of these tentacles are covered with numerous helically wound rings of 

 nematocyst-cells. For the greater part of their length the tentacles are quite straight and stiff. 

 Near the outer end, however, there is a small, pad-like, nematocyst-bearing, adhesive organ 

 upon the aboral surface of the tentacle (plate 45, fig. 3) and at this point there is a sharp bend 

 at right angles to the direction of the main shaft of the tentacle. These adhesive pads serve to 

 anchor the medusa to seaweed, etc. The tentacles all project from the sides of the exumbrella 

 at a short distance above the bell-margin. The entodermal roots of the tentacles project inward 

 into the gelatinous substance of the bell to the ring-canal (plate 45, fig. 2). There are typically 

 half as many lithocysts as there are tentacles. The lithocysts are situated on the bell-margin 

 between the tentacles. According to Perkins, the lithocysts bear a very definite relation, both 

 in place and time of development, to the tentacles. This will be discussed in the account of 

 the growth of the medusa. 



Each vesicular lithocyst contains a single, small club which bears a spherical concretion. 

 The velum is well developed. There are 4 straight, narrow radial-canals. The manubrium 

 is spindle-shaped and cruciform in cross-section. It does not extend quite to the velar opening. 

 There are 4 recurved, crenated lips. There is a small peduncle, down which the radial-canals 

 extend into the stomach. The 4 gonads are developed upon the greater part of the 4 radial- 

 canals. They are ribbon-like and extend from the base of the peduncle to a point near the 

 circular canal, but they never touch the circular canal. Each gonad is longer than the portion 

 of the canal upon which it lies and is therefore thrown or reflected into a series of sinusoidal 

 folds to one side and the other of the radial-canal (plate 46, fig. 3). The entoderm of the gonads, 

 radial tubes, and circular canal is rich brown. The entoderm of the manubrium is of a darker 

 brown, marked near the 4 angular perradial edges with pink. The lips are white. There is a 

 brilliant emerald or grass-green pigment-spot in the entoderm at the root of each tentacle near 

 the circular canal. 



This medusa was first described by Murbach from the "Eel Pond," a small body of salt 

 water, of about 5 acres in area, at Woods Hole, Massachusetts. It has continued to reap- 

 pear each summer in large numbers in this small pond since its discovery in 1894. The 

 medusa has been found occasionally in Woods Hole Harbor outside of the Eel Pond, and it is 

 reported also from Noank, Connecticut, and from Hadley Harbor, Muskegat Island (Mur- 

 bach). 



Professor Perkins has given a graphic account of the normal habits of Gonionemus, which 

 is so attractive that I can not refrain from quoting it. 



"Gonionema is a very attractive feature of the Woods Hole fauna. Its exquisite glassy 

 umbrella, marked with a cross of yellow or brown by the four radial-canals and the gonads, a 

 brilliant row of closely-set spots of gleaming phosphorescent green outlining its edge, a fringe 

 of delicate streaming tentacles strung with beadlike clusters of thread-cells, are all more or less 

 familiar to many American biologists. 



"On cloudy days or toward nightfall the medusa is very active, swimming upward to the 

 top of the water, and then floating back to the bottom. In swimming it propels itself upward 

 with rhythmic pulsations of the bell-margin, the tentacles shortened and the bell very convex. 

 Upon reaching the surface the creature keels over almost instantly, and floats downward with 

 bell relaxed and inverted and the tentacles extended far out horizontally in a wide snare of 

 stinging threads which carries certain destruction to creatures even larger than the jelly-fish itself. 

 " Gonionema continues this fishing, with little respite, all day long in cloudy weather. Occa- 

 sionally it fastens itself to a blade of eelgrass or some other object near the bottom, or stops 

 midway in its course with tentacles extended. In this position it is well-nigh invisible, but a 

 deadly foe to small fish or crustaceans which cross in its path." 



