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PACIFIC SCIENCE, Vol. IX, July, 1955 
at the point where the gonads contact the 
surface (behind the buttresses and immedi- 
ately below the marginal band), and in the 
male groups of sperm cells were in these 
pores, and in one female a large ripe egg was 
half in and half out of one of the pores. In the 
immature specimen there is a gradual change 
in cell type from the ectodermal cells to the 
gonadial cells at the points where the gonads 
reach the margin. There is no mesoglea sep- 
arating the ectoderm from the gonad at this 
point nor is there a definite epithelial layer 
covering the gonad as indicated by Komai. 
The conclusion from this is inescapable and 
obvious that the gonad is ectodermal and is 
derived as an ingrowth of epithelial ecto- 
dermal tissue into the endoderm, carrying 
with it the mesoglea which encases the adult 
gonad. 
The matter of the presence of a velum is a 
very difficult point. The hydrozoan velum is 
a solid structure consisting of two layers of 
ectoderm separated by mesoglea. The meso- 
glea may, however, be nearly nonexistent 
judging from illustrations of velar anatomy 
of many hydrozoans and indeed the demon- 
stration of mesogleal tissue in an animal like 
Hydra is not simple. Komai has interpreted 
what Carlgren and Dantan have called velum 
as tentacles and described and drawn them 
as being composed of a core of polymorpho- 
nuclear endodermal cells covered by epithelial 
cells. I find two details wrong with this con- 
clusion. First the polymorphonuclear cells are 
not endodermal, but are ectodermal and sec- 
ond these are not tentacles but are part of the 
velar sheet, two cells thick, the cells of the 
oral side being thin, flat epithelial cells while 
the aboral surface is the layer of polymorpho- 
nuclear cells. I cannot determine if there is 
any mesoglea present between these cell lay- 
ers. In the slides I have examined some sec- 
tions, through tightly rolled lappets, were 
rather similar to Komai’ s figure 9 (Komai 
1939, p. 240), but this appearance is the 
result of the rolling and folding of the velum. 
In expanded lappets there clearly are no adoral 
appendages which could be interpreted as 
Komai has or which occur in the position 
Komai has indicated. Indeed, I find Komai’s 
whole interpretation of the structure of the 
lappets to be incorrect. He has indicated that 
the lappets have on their oral surface a thin 
epithelial layer, next below this a layer of 
endodermal motor (muscular) cells with fi- 
brous bases, below these another layer of 
endoderm (these contain the polymorphic 
nuclei), next the thick mesogleal layer which 
supports the lappets and then finally the 
outer-most layer of thin epithelium on the 
aboral surface. I find all these structures but 
cannot subscribe to his interpretations. There 
is a thin epithelial layer overlying a muscular 
layer on the oral surface of the lappets; how- 
ever, both these tissues are ectodermal and 
are separated from the next layer, the endo- 
derm, not by the fibrous bases of the motor 
cells but by a thin fibrous layer of mesoglea. 
A double sub-umbrellar layer of ectoderm is 
not an unknown feature in the Hydrozoa, the 
innermost layer being the muscular one. That 
the layer possessing polymorphonuclear cells 
is endodermal I do not question, since this 
layer can be traced through the serial sections 
and its confluence with the endoderm of the 
coelenteron is clear. Also the polymorphic 
nuclei gradually become regular ovoid nuclei 
near the base of the lappet and become iden- 
tical in structure to those of the endoderm 
proper. The thickened aboral layer of meso- 
glea is a clear-cut structure as is the outer, 
aboral ephithelium and on their nature Komai 
and I agree. In my slides it is clear, however, 
that it is this latter, ectodermal, layer which 
gives rise to the cells with polymorphic nuclei 
of the velum and there appears to be no 
possibility that this could be an endodermal 
tissue in spite of the similarity of its nuclei 
and those of part of the endoderm. 
Komai dismissed the absence of septa, 
gastric filaments and cells in the mesoglea as 
larval features. In the Scyphozoa the septa 
are well developed in the scyphistoma larva 
while the cellular mesoglea and gastric fila- 
