430 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 



The young hydranths and the medusae are budded off above the 

 collar, but they soon become entirely sheathed in perisarc by the 

 growth of the stem. The pale yellowish-red hydranths, are very 

 similar to those of Tubularia (Allman) and the hydroid is so similar 

 to Dendroclava Dohrnii recently described by Weismann, that they 

 undoubtedly belong in the same genus. 



'The little medusa remains attached to the stem, as shown in PL 

 37, C, for some time after the rupture of its capsule of perisarc. At 

 this time it is nearly spherical and covered with large conspicuous 

 ectoderm cells. Its eight short tentacles are thrown backwards in 

 contact with the outer surface of the bell, and their tips are hooked or 

 bent upon themselves in a very characteristic manner, which is shown 

 in the figure. This position of the tentacles renders the bulb at the 

 base, with its ocellus, very prominent. 



" The medusa when set free, PL 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." When it is 

 relaxed after a pulsation the bell is nearly globular, but it is length- 

 ened and emarginated at each pulsation, so that it exhibits rhythmical 

 changes of outline. Figure 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 the medusa is at rest the height of the umbrella is about equal 

 to its diameter, and the shape is that of a spherical segment almost 

 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 at an acute angle near the middle of the ten- 

 tacle. The four interradial tentacles, when thus extended, lie nearly 

 in the plane of the velum, while the four perradial tentacles are carried 

 a little loAver. This peculiar bending and alternation of the tentacles, 

 which is very characteristic, is well shown in figure 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 resem- 

 blance to these hydroids, which Turritopsis exhibits, seems to be 

 a larval, or hydroid characteristic, for I have failed to observe any- 

 thing of the sort in older medusae. The tips of the extended ten- 

 tacles are slightly clavate, each with a spot of dark orange pigment. 



