No. 395.] PENNARIA TIARELLA McCRADY. 863 
roundish bodies. In their reactions to stains they show that 
they are probably the nuclei of the ectodermal cells. The pro- 
toplasm is more or less vacuolated ; large, round vacuoles occur- 
"ring scattered through it. This is especially noticeable in older 
hydrorhiza. In part, the appearance of all the tissues will 
depend largely upon the age. A young hydrorhiza differs from 
an old one in the degree to which degeneration of the parts has 
gone. On the lower side, or side adhering to the substratum, 
the ectoderm is only distinguishable as a narrow layer in which 
there are a fewscattered granules. Otherwise it appears to be 
structureless. It is not more than one fourth as thick here as 
it is on the upper side.. In the young hydrorhiza, which is 
creeping over the eel-grass, the ectoderm contains cells which 
are very similar to the cells in the endoderm. The same is 
also true of the ectoderm in the upper side during this stage. 
The endoderm exhibits a cellular structure, although the cell 
walls are very indistinct. The cytoplasm has been broken up 
into small, round bodies, and the nuclei are much smaller here 
than elsewhere in the endoderm of the hydroid. 
Flydrocaulus. —The perisarc in section in a young hydro- 
caulus is semi-transparent or opaque. It is thicker than when 
mature. In the mature hydrocaulus the perisarc is thinner, and 
is darker adjacent to the ectoderm than in the hydrorhiza. The 
ectoderm shows signs of cellular structure in the position of the 
nuclei and the presence of cell walls in places. As the hydro- 
caulus is examined nearer the hydranth the nuclei are found to 
belong to definite cells with definite cell walls. If the endoderm | 
is examined in a typical section, it is very materially different 
from the endoderm in the hydrorhiza. The nuclei are larger 
and more prominent. The cytoplasm is homogeneous except 
for the presence of food particles. 
Hydranth.— The hydranth is wholly devoid of perisarc, which 
terminates somewhat abruptly just below it. The cells of the 
ectoderm possess distinct nuclei and cell walls, but only a small 
amount of cell substance, judged by the reaction to stain. The 
cells of the endoderm are arranged in the hydranth in groups, - 
usually there are four of them. The most conspicuous of these 
are the digestive cells. They are much larger than the other 
