62 
direct contact with the bases of the worm’s 
tentacles, which is the normal position, al- 
though its location inside the tube was un- 
usual, The hydrorhizal net of this specimen 
started on the outside of the tube, traveled up 
the tube to its lip, and then down inside the 
tube to the ring of gastrozooids. The location 
of this colony suggests that the individuals 
may have migrated from the old lip to the 
new position, although it may be that the 
hydroid settled upon the tube after it had 
been damaged. It is not known how ”Lar” 
responds as its host lengthens and enlarges 
its tube, if indeed the worm does so, after a 
colony has established itself upon it. 
The individual gastrozooids are never more 
than 1 millimeter tall and are rather stout, 
being 0.2 to 0.3 millimeter thick. The nema- 
tocyst pad on the "head” is well developed 
and is devoid of pigment, as is the ectoderm 
in general. The endoderm is brown. 
The gastrozooids are connected to one an- 
other via a network of anastomosing, naked 
hydrorhizae, these hydrorhizae not exceeding 
0.1 millimeter in diameter. 
Tentacleless gonozooids occur as scattered 
individuals arising from the hydrorhizal net. 
The gonozooids are about 1 millimeter tall 
and bear a capitate, mouthless tip which is 
studded with nematocysts. From 2 to 12 me- 
dusa buds are borne on each gonozooid. They 
arise at irregular intervals slightly above the 
mid-point. The medusa buds show four spots 
of brownish-red pigment which correspond 
to the developing first order tentacle bulbs 
and also a brownish pigm.ent mass represent- 
ing the developing stomach. Medusa buds 
were seen pulsating on several colonies, but 
none were released before the specimens were 
preserved. 
The nematocysts of th e hydroid were studied 
and found to be as follows: 
Macrobasic euryteles 20 — 28X8 — 
Large microbasic 
euryteles 12 — 24X4 — 6^i 
PACIFIC SCIENCE, Vol. VIII, January, 1954 
Small microbasic 
euryteles 5— 8X2— ^/j, 
Desmonemes 6— 9X3— 4m 
The tentacles seem to possess only the small 
microbasic euryteles, whereas the macrobasic 
euryteles and desmonemes occur only in the 
cap of the gastrozooid. The large microbasic 
euryteles occur on the body of the polyp as 
well as in the cap. The body of the polyp 
also has small microbasic euryteles. Figure 
5/-/ illustrates the above nematocysts. 
THE HOSTS: This species (P. occidentalis) has 
been found on two species of sabellids, Pota- 
milla neglecta and Pseudopotamilla intermedia. 
These worms were kindly identified by Dr. 
Olga Hartman of the Allan Hancock Founda- 
tion. Sabellids are not well represented in the 
intertidal region near La Jolla, and only a few 
small specimens of Pseudopotamilla intermedia 
have been collected. These did not have any 
"Lar” upon them. The material upon which 
the hydroid was found was taken from 40 to 
50 feet of water, as mentioned earlier. 
DISCUSSION 
A group of organisms, such as is repre- 
sented by the genus Prohoscidactyla, presents 
a number of interesting problems to the bi- 
ologist. One of these problems concerns the 
matter of symbiosis, more specifically com- 
mensalism in this instance, and a second 
concerns the special life history of this group 
with the unavoidable complications encount- 
ered where the two phases of the life history 
are adapted to, or are adapting to, very dif- 
ferent environmental factors. 
In the Hydrozoa the tentacles, with their 
associated nematocysts, represent the primary 
tools for protection and food gathering, and 
there are few hydrozoans which can be pointed 
to as possessing reduced numbers of tentacles. 
Protohydra and Tetraplatia are curious among 
the Hydrozoa, indeed among the Coelen- 
terata, in that these free living animals possess 
no tentacles at all. On the other hand, among 
the symbiotic Hydrozoa a rather large group 
