BROOKS AND RITTEXHOUSE: ON TURRITOPSIS. 431 



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 when the medusa is first set free, 

 and for several days afterwards, is simple and circular, and the endo- 

 derm 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 subunibrella in the course of 

 the radial canals, as is shown in figure 7. This thickening of the 

 endoderm of the proximal ends of the radiating tubes, where they 

 join and open into the stomach, is characteristic of Turritopsis, and, 

 in specimens a week old, (fig. J) the peduncle that is thus formed is 

 about equal in length to the proboscis proper, which is suspended at 

 the lower end of a peduncle consisting of four great masses of carti- 

 lage-like endoderm cells. These four hollow tubes of cartilage are 

 the endodermal portions of the proximal regions of the radial canals, 

 which meet each other in the central axis. The singular struc- 

 ture which is thus formed is quite unlike anything that has been 

 described in other genera. Some authors have regarded it as an 

 ordinary gastrostyle or gelatinous prolongation of the subumbrella. 

 It is not a gastrostyle but a peduncle formed of endoderm cells. As 

 the young medusa grows, the proximal ends of the radiating tubes 

 are drawn down into the cavity of the umbrella, as is shown in figure 

 H, 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 cavities of the four 

 tubes. In the adult, figures /, J, K, this body almost entirely fills 

 the upper half of the cavity of the bell. 



In a medusa a week old, figure H, the four oral lobes or lips are 

 present, and are covered by the stalked bunches of lasso-cells that 

 have been described in the adult by McCrady and others. 



At about this time traces of the reproductive organs make their 

 appearance in the walls of the proboscis just below the lower ends 

 of the masses of endoderm cells. The tentacles, at the stage shown 

 in figure H, are still carried in two cycles, the interradials being higher 

 than the perradials. There are only eight, and no more were devel- 

 oped in the medusae which I reared from the hydra, although I cap- 

 tured many specimens in the same stage, and at all the following 

 stages up to maturity. 



In specimens from one to two weeks old the lower surface of the 



