236 TOILERS IN THE SEA. 



droids ; and here we may fall back upon what we 

 have already written for many of the facts, to avoid 

 repetition. The hydroid zoophytes, as they are often 

 called, will hereafter be described, inasmuch as 

 concerns the " Horny House-builders." These may 

 be taken to a great extent as a type of the Hydroid 

 coral animals, since, in both, the common fresh-water 

 Hydra furnishes a good illustration of general struc- 

 ture. When speaking or writing of corals, allusion 

 is often made to a group called Millepores, in which 

 the structure of the animals recedes from that of the 

 Actinoid corals, to which our space has hitherto been 

 devoted. These animals are slow to make their ap- 

 pearance ; and, indeed, it has not long been known 

 to what precise position in the animal kingdom they 

 should be assigned. At length, however, the American 

 naturalist, Agassiz, determined their character, which 

 seems to be generally accepted, that they are Hy- 

 droids, allied to our own Coryne, but less perfectly 

 developed. The animal appears to be only a fleshy 

 tube, with a mouth at the top, and, in place of the 

 tentacles, a few small rounded prominences, four of 

 which are somewhat the largest. The corals are 

 large and stony, punctured all over the surface with 

 small rounded holes, through which the animals pro- 

 trude themselves, so that the corals themselves may 

 be readily distinguished from those of the anemone 

 type. As these corals are very abundant, and con- 



