EEPOBT ON THE HYDBOIDA. 
9 
Halecium, Oken. 
Halecium, Oken, Lebrbucli d. Naturgesch., 1815. 
Generic Character. Trophosome . — Colony dendritic, with fascicled or monosiphonic 
stem. Hydrothecae replaced by shallow pedunculated cups (hydrophores), too small to 
receive the hydranths in retraction. Sarcothecae not present. 
Gonosome. — Gonophores adelocodonic. Gonangia with terminal or lateral orifice. 
There is no more natural and definitely marked genus among the Calyptoblastic 
Hydroids than that of Halecium, with two or three others which must be united with 
it in one and the same family. The hydrotheca so well developed in other Calyptoblastea 
is in these genera rudimental, being reduced to the condition of a peduncle, with its 
distal, end expanded so as to form a very shallow membranous cup, quite incapable of 
receiving the hydranth, even in its state of extreme retraction. The term hydrotheca is 
thus inapplicable to it, and that of “ hydrophore ” may be conveniently used to designate 
the peduncle with its terminal expansion or “ limbus.” 
The primary hydrophore is always immovably fixed by its base to the hydrocaulus, 
but in most cases it becomes, with the growth of the colony, prolonged by the successive 
formation of new hydrophores which originate within the limbus of the preceding one, 
and the primary hydrophore is thus extended by a succession of similar segments piled 
one on the other in a single continuous series, which is strongly suggestive of a telescope 
with its tubes drawn out. 
The limbus of the hydrophore is in almost every instance ornamented by minute 
refringent puncta which run round its walls in the form of a circular wreath. What may 
be their meaning is unknown, but that they are not without significance may be assumed 
from the constancy of their occurrence. They are found in the most widely separated 
species, in species in which the limbus is least developed as well as in those in which 
this part is most obvious, and occur not only in Halecium proper but in forms which, 
though belonging to the same family, must be placed in different genera. Among the 
various species which I have had an opportunity of examining, there is only one 
( Halecium cymiforme, PI. VII.) in which I have failed to detect them. 
The gonangia of Halecium spring in some cases from the proximal or primary segment 
of a hydrophore, sometimes from the side of a branch, and sometimes as a continuation of 
its distal end. The orifice is sometimes terminal, sometimes lateral, and there is probably 
always a difference of form between the gonangia of male and female colonies. In 
some cases the blastostyle develops from its expanded summit on the outer side of the 
gonangium a pair of well-formed hydranths with tentacles and mouth, a remarkable 
phenomenon of which we have no other example in any Hydroid. 
In the family of the Haleciidse must be included a beautiful Hydroid obtained by the 
Challenger, and forming the type of a new genus, Diplocyathus (PI. VIII.). In the same 
(zool. chxll. exp. — part lxx. — 1888.) Aaaa 2 
