Tetraplatia — Hand 
347 
tion from the aeginid line, or that Tetraplatia 
has evolved directly from primitive narco - 
medusan ancestors at a point so early that the 
true medusoid form had not yet been estab- 
lished. If this latter is true, then Tetraplatia 
must represent an early evolutionary experi- 
ment in developing a more complex organism 
than the presumed actinular ancestors of the 
trachyline medusae. Another suggestion could 
be that Tetraplatia represents a neotenic ac- 
tinula, and as such might better be called 
aberrant than primitive. Regardless of which 
of these hypotheses one might subscribe to, 
I feel that the unusual characteristics of Tetra- 
platia place it in a position between the two 
recognized orders of trachyline medusae and 
that it is morphologically so distinct that it 
deserves to be placed in an order of its own. 
I therefore subscribe to the classification pro- 
posed by Carlgren (1909) and recognize Tetra- 
platia as a pteromedusan. Further light may 
be shed on the actual ancestry of Tetraplatia 
once its development and complete life his- 
tory are known. 
The distribution of Tetraplatia in the East- 
ern Pacific, as indicated in Figure 4, presents 
a pattern in the northern latitudes which one 
might predict on the basis of known ocean 
currents. That is, this animal seems to live in 
the water which slowly moves in a clockwise 
direction and makes up the North Pacific 
Current. On this basis one can predict that 
Tetraplatia can be expected to be found 
throughout this whole current system in- 
cluding the Kuroshio and the North Equa- 
torial Current. Komai (1939) has already 
reported four specimens from Japan. Further 
collecting alone will test this suggestion. 
However, if one examines Figure 4, it will be 
seen that there is a considerable gap between 
the most southerly point at which Tetraplatia 
was collected and the main area from which 
most specimens were taken. That is, no spec- 
imens were collected between 5° south lati- 
tude and 20° north latitude. The area covered 
by the lack of specimens was sampled by the 
Scripps Institution cruise "Shellback” and the 
specimens taken at 20° N. and 5° S. were 
from that expedition. Although not all of the 
plankton tows taken on "Shellback” have 
been examined by the writer, 18 samples 
spaced throughout the gap have been exam- 
ined and no Tetraplatia were found. The tows 
in this area were oblique hauls with a one- 
meter net fishing from 300 meters depth to the 
surface. The presence of one specimen of 
Tetraplatia at 5° S. suggests that Tetraplatia 
is present in the cooler waters of the South 
Pacific, since this specimen was from the 
northern end of the Peru Current, a cold 
water mass moving in a counter-clockwise 
direction. The gap between this cold water 
mass and the cold water of the North Pacific 
is filled with the warmer waters of the Equa- 
torial Counter Current and Tetraplatia may be 
unable to live in this area of warmer water. 
This, perhaps, explains the absence of Tetra- 
platia in the area under consideration, al- 
though a possible alternative solution might 
be in the fact that colder waters underlie the 
area and Tetraplatia may be found there if 
samples are taken from the deeper waters. 
If this is so, then the lack of Tetraplatia may 
be an artifact of sampling and the result of 
tropical submergence as this animal stays in 
the colder waters which may offer to it a more 
suitable environment. 
CONCLUSION 
Tetraplatia volitans is considered, by the 
author, to be a highly specialized and diver- 
gent trachyline medusan of narcomedusan 
ancestry which has evolved a trachymedusan 
sense organ. Its adult morphology seems to 
represent a compromise between the struc- 
tures of actinula larvae and adult narcome- 
dusans. Its highly specialized lappets and new 
features such as the flying buttresses and 
enclosed adoral lithostyles represent unique 
characters. In order to give proper weight to 
the unusual nature of this animal, Carlgren ’s 
order of Pteromedusae is recognized as a valid 
classification. Tetraplatia is nearly worldwide 
in distribution and may be characterized as a 
