BROOKS AND RITTENHOUSE: 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 subumbrella 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 gastrostvle 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 I, 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 Id, 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 
