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PACIFIC SCIENCE, VoL IX, July, 1955 
Contrary to Viguier’s opinion Weill felt that 
the variability in the size of the nematocyst 
capsules was insufficient to recognize more 
than one category— atrichs. In the present 
state of our knowledge the Narcomedusae, 
the group to which Tetraplatia was assigned 
by Weill, may be stated to have a mono- 
cnidom of atrichous isorhizas (atrichs). 
I have examined twelve specimens of Tetra- 
platia volitans to determine the cnidom of this 
species as it exists in the eastern Pacific. 
Nematocysts are rare or absent from most of 
the body and are largely confined to the 
nematocyst tracts, although near the distal 
end of the manubrium the tracts become 
diffuse and nematocysts are generally dis- 
tributed over this area. Only one type of 
nematocyst has been found in the tissues of 
Tetraplatia , this being an atrichous isorhiza, 
although two very obvious size categories are 
present. The smaller capsules appear to con- 
tain a coarser thread than the larger, and the 
thread in the larger capsule has many more 
coils than the smaller. Figure 8 illustrates the 
appearance of these atrichs. The capsules are 
all very nearly spherical, with some tendency 
to develop a low conical shape toward the 
opercular region, at least in the smaller ones. 
The large atrichs are restricted to the manu- 
brium in their occurrence while the smaller 
occur throughout the nematocyst tracts. Dan- 
tan (1925) indicated that he too found a large 
and small type of nematocyst, although he 
gave very little indication of their structure. 
The sizes of the two categories of atrichs 
were as follows: 
Small— 8-12 n diameter 
Large — 18-22 /i diameter 
Komai (1939) also found two sizes of nema- 
tocyst, one 20 n in diameter, the other 8 /z. 
He did not indicate what type nematocyst 
these were but his sizes agree well with what 
I have found. 
Statocysts 
Both Viguier (1890) and Dantan (1925), 
in their illustrations of whole Tetraplatia , have 
indicated the positions which the statocysts 
occupy. Carlgren (1909) has also described 
their location although he did not indicate 
their positions by figures. These writers seem 
to agree that the statocysts are located on 
the oral sides of the lappets, one in the mid- 
dle of each of the two major lobes of the 
lappets at the junction of the velum and the 
lappet proper. Dantan had some difficulty in 
locating the statocysts, due to the more or 
less contracted and coiled- up condition of the 
flaps, and had to rely on sections to locate 
them. A number of the Tetraplatia from the 
Pacific have been examined in which the lap- 
pets were fully extended and in excellent 
states of preservation. As was remarked ear- 
lier, the term "bilobed” as applied to the 
lappets, is something of a misnomer since 
the lappets actually are divisible into at least 
eight terminal lobes. Moreover, the statocysts 
are not located at the midpoints of the two 
halves of the lappets, but are to be found just 
to the outer side of the second bifurcation 
of the supporting core of the lappets, as 
illustrated in Figure 3. The statocysts occur 
in a raised knob of tissue, and in sections it 
appears that the lithocytes have been derived 
from the endoderm. Figure 2 illustrates a 
section of a lappet through a statocyst. 
In considering the structure of the lappets 
one feature appeared which was of consider- 
able interest. This is the morphology of the 
nuclei of the velar and endodermal cells. The 
nuclei of the velum show a remarkable flat- 
tened and multilobed polymorphic structure, 
which is reminiscent of the nuclei of poly- 
morphonuclear leucocytes. This is also true 
of some of the endodermal nuclei of the 
lappets which show a gradual transition from 
ovoid nuclei near the lappet base to the flat- 
tened, polymorphic type in the distal lappet 
endoderm. Dantan (1925) also noted these 
curious nuclei and Komai (1939) concluded 
that all of the cells possessing these poly- 
morphic nuclei were endodermal. 
The fully extended lappets of three spec- 
imens were constructed as is shown in Figure 
