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( 637 ) 
gasterozooids but in every case the gonozooid arises, not from the 
stolon but from the base of a gasterozooid (fig. 7) in close proximity 
to the stolon or basal coenosare. 
Stolon. 
The stolons are tubular in structure (fig. 3). Their walls consist 
of superficial ectoderm and a lining of endoderm separated by a 
structureless lamella, the mesogloea. The ectoderm exhibits no traces 
of a perisare. It possesses a few scattered nematocysts of the smaller 
kind .008 mm. > .005 mm. (vide infra). 
Basal Coenosarc. 
The basal coenosare is formed by the anastomosing of stolons 
running over the parts of the autozooids immediately behind the 
tentacles. When two stolons run together or cross, the ectoderm of 
the dividing walls disappears so that the upper ectoderm of one stolon 
becomes continuous with the upper ectoderm of the other and the 
lower ectoderm of the one becomes continuous with the lower ecto- 
derm of the other. Thus the basal coenosare of the hydroid consists 
of superficial ectoderm and lower ectoderm separated by endodermal 
tubes (fig. 5). The structure arrived at is thus the same as in the 
coenosare of Hydractinia except that Ptilocodium has no chitinous 
skeleton. 
Nematocysts. 
The hydroid possesses two sets of nematocysts. The larger kind is 
found in the dactylozooids. Here the nematoeysts are oval in shape 
and measure .017 mm. & .008 mm. The smaller kind occurs in the 
ectoderm of the basal coenosare and of the gonozooid, the size of 
these nematocysts being .008 mm. > .005 mm. 
Gasterozoovds. 
Gasterozooids occur at frequent intervals on the basal coenosare 
and are sessile (fig. 7). They vary in size from .213 mm. high and 
106 mm. broad, to .373 mm. high and .026 mm. broad. They are 
much reduced in structure. There are no traces of tentacles. The 
zooid is simple and sac-like; the mouth is a simple pore leading 
from the exterior into the cavity of the zooid. The gasterozooids 
show no nematocysts. The endoderm cells near the mouth of the 
gasterozooids are comparatively short whilst those lining the remainder 
of the gastral cavity are long and narrow. The material is not sufli- 
ciently well preserved to make out clearly the histological structure 
of the cells but it seems probable that the digestive functions are 
preformed by the long, narrow cells of the basal half of the gaste- 
rozooids. 
