64 
PACIFIC SCIENCE, VoL VIII, January, 1954 
amined by the writer have not possessed 
ocelli. The species Prohoscidactyla polynema of 
Foerster is referred to the animal described 
from the Atlantic by Hartlaub (1917) as Po- 
chella polynema. This medusa has no cnido- 
thylacies (centripetal canals of Foerster), and 
each primary radial canal gave off about six 
lateral branches on each side. These characters 
would make this an unusual species of Pro- 
hoscidactyla, if, indeed, it is one at all. 
The following four species of Prohoscidac- 
tyla now have been described for both the 
hydroid and medusoid stage of the life 
history. 
P. stellata (Forbes) (Hydroid Lar sabel- 
larum Gosse) 
P. flavkirrata Brandt 
P. occidentalis (Fewkes) 
P. circumsahella n. sp. 
This leaves an ample number of hydroid 
stages to be described, such as the several 
which presumably live along the Atlantic 
coast of the United States. At least one of 
these hydroids is known to occur in Long 
Island Sound (E. S. Deevey, in litt.). This 
hydroid will probably be identified with the 
medusa P. ornata (McCrady) when a critical 
study is made. 
In considering a group of species it is al- 
ways interesting to see if a key to their evolu- 
tion or the manner in which they are related 
can be found. A group such as Prohoscidactyla 
has no fossil record to help us, and today 
they are widely distributed. At least four spe- 
cies occur in the Pacific and an equal number 
in the Atlantic. This does not immediately 
suggest a key to their evolution. 
If the hydroid stages of Prohoscidactyla are 
compared, certain relationships are suggested. 
P. stellata and P. flavkirrata both have gono- 
phores borne close to or upon the gastro- 
zooids, whereas neither of the other two spe- 
cies whose hydroids are known is so con- 
structed. Also, P. stellata commonly produces 
six-rayed medusae, whereas P. flavkirrata may 
have four, five, six, or eight radial canals. 
This suggests a relationship between these 
two, as the other two species seldom if ever 
produce anything other than four-rayed me- 
dusae. The nematocysts of the polyp stages 
of P. stellata and P. flavkirrata also resemble 
each other very much, but they also closely 
resemble those of P. circumsahella and P. 
occidentalis. 
The gonozooids of the four species do not 
give us much help beyond their location on 
the colony. In P. stellata four medusa buds 
are usually produced, whereas P. flavkirrata 
may have up to eight buds; P. circumsahella 
seems never to produce more than four, where- 
as P. occidentalis produces up to 12. 
The adult medusae of Prohoscidactyla seem 
to offer the best evidence to their paths of 
evolution. P. stellata and P. flavkirrata have 
already been mentioned as having six or a 
variable number of radial canals, respectively, 
and it should be mentioned that P. mutahilis 
of the South Atlantic is even more variable 
than these two species. Of the three species 
of the present report, the northern species 
(P. flavkirrata) is largest and has more ten- 
tacles than its two southern relatives. 
The nematocysts of the medusae are known 
for only the three species reported herein. Of 
the three, P. occidentalis is the most distinctive 
in that three sizes of macrobasic euryteles are 
present of which one, the medium size, is 
very unusual in shape. P. flavicirrata and P. 
circumsahella, on the other hand, possess only 
two sizes of macrobasic euryteles. If the ne- j 
matocysts of the medusae are used to indicate • 
relationships, it appears that the latter two | 
species are more closely related to each other | 
than to P. occidentalis. In summary, then, it 
appears that P. stellata and P. flavicirrata are j 
closely related, the latter being also near P. 
circumsahella. P. occidentalis is somewhat more |! 
distantly related to these others in both its 'i 
morphology and its geographic location. ! 
Another interesting medusa, P. ornata (Me- j 
Crady) var. stolonifera Maas, has been reported j 
from the Pacific at Acapulco harbor (Mexico) ! 
by Bigelow (1909) and from the Malay Ar- : 
