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 colon}' 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 is a 

 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. 



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 dactylozooids, with tentacles and 

 stinging-cells. These seem to be the guard-polyps of the community. 

 The cups occupied by the zooids 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 zooids. The living animal 

 occupies only the outer, open space. (Plate XLIV.) 

 9 



Trachynema digitale. 



