A Study of the Structure, Affinities, and Distribution of Tetraplatia 
volitans Busch (Coelenterata: Hydrozoa: Pteromedusae ) 1 
Cadet Hand 2 
INTRODUCTION 
The Hydrozoa include a large number of 
curious animals, not the least unusual of 
which are the tetraplatians. Two species of 
Tetraplatia have been described, T. volitans 
Busch (1851) and T. chuni Carlgren (1909). 
These hydromedusans are so highly modified 
in form that their true nature is seldom rec- 
ognized when they are first encountered. 
Tetraplatians are elongate (to 9.5 mm.) 
medusans of bipyramidal form with ciliated 
outer surfaces. The mouth lies at one ex- 
tremity whereas the other end represents the 
aboral tip of the bell. In cross section the 
aboral region may be nearly cylindrical, quad- 
rangular or octagonal while a cross section 
of the manubrium (oral end) may be circular 
to quadrangular. A constriction occurs near 
the middle of the body. In Tetraplatia volitans 
(Fig. 1) this constriction is bridged by flying- 
buttresses connecting the angular corners of 
the manubrium to the four major angles of 
the bell above the groove. These buttresses 
are absent in T. chuni. In the constriction and 
between the buttresses in T. volitans , and in 
a corresponding position in T. chuni , arise 
1 Contribution from the Scripps Institution of 
Oceanography, New Series No. 769. Manuscript re- 
ceived November 22, 1954. 
2 The Department of Zoology, University of Cali- 
fornia, Berkeley, California. 
four, bilobed, swimming appendages, the 
wings or lappets. Each lobe of each lappet 
has a statocyst. These lappets have a solid 
endodermal core (see Fig. 2). At the free 
extremity of the lappets there is a thin sheet 
of ectoderm which is interpreted as the velum; 
this velar structure may be continuous around 
the body in the constriction although not all 
specimens seem to show this. The lappets 
commonly have been called bilobed; how- 
ever, at their free end 8 or 10 small velar 
projections can be found, four or five to each 
lobe. These projections are the result of at 
least three bifurcations of the supporting 
structure within the lappet, the first producing 
the bilobed appearance of the contracted lap- 
pet and the latter ones leading out to the 
terminal velar projections of the lappets (Fig. 
3). The surface of tetraplatians is marked by 
ridges, which are tracts of nematocysts; com- 
monly the bell has eight tracts, four in the 
axes of the buttresses and four in the axes of 
the lappets, although those in the lappet axes 
may be absent. The manubrium never has 
more than four rows of nematocysts and these 
always are in the buttress axes. 
Internally tetraplatians consist of a large, 
essentially quadrangular, gastrovascular cav- 
ity. The coelenteron extends into the but- 
tresses from both the oral and aboral halves 
of the body. It has been reported that at the 
midpoint of the buttresses there may be a 
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