No. 395.] PENNARIA TIARELLA McCRADY. 867 
reproductive elements. Another layer is yet to be formed. 
‘As the reproductive elements continue to develop, a thin 
delicate membrane is produced which seems to be a differ- 
entiation from these cells (see Fig. 5). In the male it is always 
very delicate, while in the female it becomes more prominent. 
The manubrium has grown through the cells of the ectoderm 
while these other changes have been going on, and divides them 
into two equal parts when seen in section. The cells of the 
manubrium become differentiated until they present the same 
appearance as the endodermal cells in the hydranth. The cells 
of the endoderm which form the second layer of the bell undergo 
the following changes: the endoderm becomes much thicker in 
four regions equally distant from each other. This thickening 
gives rise to the chymiferous tubes by 
a process of cleavage and not by a 
fusion of two layers, as is the usual 
way. The splitting begins at the point 
where the endoderm turns in from the 
bell to form the manubrium (Figs. 4 
and 6). This process is continued 
until there is a canal extending down 
to the lower margin of the Medusa. 
It is much larger at the lower end and 
extends farther around than do those same canals halfway up. 
The endoderm between the chymiferous tubes has not under- 
gone any perceptible change during their growth. In P. cavo- 
lint the chymiferous tubes are much larger than in P. tiarella, 
probably because they are functional for a longer time. In this 
hydroid the Medusze are not set free and these tubes are still 
found in the mature Medusa. 
The tentacles are rudimentary and never develop. They 
consist simply in a thickening of the outer layer of the ecto- 
derm. They are found at the end of the chymiferous tubes 
and are four in number. 
The sense organs originate from the endoderm at the blind 
end of the chymiferous tubes. They do not show any very 
definite structure. In one there could be distinguished a 
central portion, which was more transparent than the rest, and 
