392 W. K. BROOKS ON THE LIFE-HISTORY 



shown in the figure. This position of the tentacle renders the bulb at the base, with its 

 ocellus, very prominent. 



The medusa when set tree, PI. 37, figs. D and E, has eight tentacles, a thin 

 globular bell, and a short simple proboscis. When the animal is in active motion the 

 tentacles are contracted, bent into hooks and thrown back against the umbrella, as shown 

 in fig. D, and at each pulsation the bell is lengthened and emarginated during con- 

 traction, but when relaxed it is nearly globular. Fig. D shows a young medusa in the 

 shape which it assumes while swimming, at each period of contraction, while E shows 

 a medusa of the same age floating in a relaxed condition. When at rest the height of the 

 umbrella is about equal to its diameter, and the shape is that of a spherical segment al- 

 most equal to a sphere. The tentacles are capable of extension to a length equal to about 

 twice the diameter of the bell, and when the animal is at rest they are stretched out almost 

 horizontally, and the distal half is bent downward a little at an obtuse angle near the 

 middle of the tentacle. The four interradial tentacles, when thus extended, lie nearly in 

 the plane of the velum, while the four perradial tentacles are carried a little lower. This 

 peculiar bending and alternation of the tentacles, which is very characteristic, is well 

 shown in tig. E, which, like all the other figures, is a careful study from life. Many 

 hydroids carry their tentacles bent so as to form two cycles, and the resemblance to 

 them which the young Turritopsis exhibits, seems to be an embryonic characteristic, for 

 I have failed to observe anything of the sort in older medusa?. The tips of the extended 

 tentacles are slightly clavate, each with a spot of dark orange pigment. The length of 

 the proboscis of the young medusa is about two-thirds the height of the umbrella, and 

 its upper and lower ends are smaller than the middle. The mouth of the medusa, when 

 it is set free, and for several days afterwards, is simple and circular, and the endoderm of 

 the oral end of the proboscis is thin; but, just below the aboral constriction, it becomes 

 very thick and cartilage-like, and the thickened area arches out into the sub-umbral sur- 

 faces of the radiating tubes, as shown in fig. 7. 



This thickening of the endodermal cells of the aboral end of the stomach is character- 

 istic of the genus Turritopsis; and in a specimen a week old, fig. 7/, the whole upper 

 half of the proboscis is made up of four great masses of very large, cartilage-like endo- 

 derm cells, which meet upon the central axis and run out for a short distance into the 

 radiating tubes, which penetrate the masses of cells on their way to the stomach, the cav- 

 ity of which lies below the cartilaginous peduncle. The singular structure which is thus 

 formed is quite unlike anything which occurs in any other genus. It has been described 

 by various authors as an ordinary gelatinous peduncle or gastrostyle, but it is not at all 

 the same as the gelatinous projection from the substance of the umbrella which, in many 

 medusa?, hangs down in the centre of the hell. 



As the medusa grows the proximal ends of the radiating tubes are drawn down into 

 the cavity of the umbrella, as shown in fig. //, until in specimens two weeks old the 

 stomach is suspended some distance below the sub-umbrella by a transparent mass of 

 large cells meeting in the central axis, and perforated by the four tubes. In the adult, 

 figs. I </, K, this body almost entirely fills the upper half of the cavity of the bell. 



In a medusa a week old, fig. II, the four oral lobes or lips have made their appear- 



