HYDROZOA 129 
cases hang in long pendent sacs from the upper part of the radial canals 
and reach nearly to the velum. Four garnet-colored eyes in club-shaped 
processes are prominent on the 
margin. The animal moves by 
jerks in straight lines, 
ORDER HYDROCORALLINA 
CALCAREOUS HYDROIDS 
The genus Millepora (“thou- 
sand pores”), which is the type 
of this order, is a colony of 
animals, like other hydroids, 
which secrete calcareous in- 
stead of horny coverings. It 
differs from true corals in that 
the members of a colony 
perform different functions, 
whereas in true corals each 
member of a community isa ax, 
complete individual. It differs _» 
also in the arrangement of 
the stony partitions, which in 
Hydrocorallina are the outside 
coverings and connecting 
canals, but in true corals are vertical partitions inside the animal, 
between the inner and outer sacs, as explained on page 114. 
Trachynema digitale. 
Genus Millepora 
M. alcicornis, elk-horn coral. This beautiful coral, which is abun- 
dant in Florida and contributes to the building of the reefs, rises in 
broad expansions, more or less lobed, and suggests by its shape the 
object for which it is named. The whole mass is porous, being traversed 
by innumerable canals. Its surface, although smooth compared with 
that of other corals, is covered with very minute pores, which are of two 
sizes. The larger ones are the gastropores, or stomach-pores, in which the 
nutritive animal lives; it has a cylindrical body, with four knob-like 
tentacles and a mouth. Placed more or less irregularly around the 
gastropores are smaller pores, the dactylopores (finger-pores), from which 
emerge slender mouthless processes, or dactylozodids, with tentacles and 
stinging-cells. These seem to be the guard-polyps of the community. 
The cups occupied by the zodids are shallow. As one animal dies, 
another succeeds it and builds a horizontal partition separating the 
new cup from the old one. Thus the stony mass increases in size by 
the progress of succeeding generations of zodids. The living animal 
occupies only the outer, open space. (Plate XLIV.) 
9 
