STRUCTURE OF CORALS. Ill 



layers are united at the mouth, and each tentacle consists of a pro- 

 longation of the layers of the body, enclosing a branch of the 

 stomach, and having many stinging or thread cells. No hard skin 

 layer is ever at any time developed in the Hydree. In none of them 

 is there a haemal or blood system, nor a neural or nerve system, and 

 aeration is effected by contact of the whole surface with the water 

 in which they live. 



These creatures increase by budding, like plants. They are also 

 hermaphrodite, being alternately propagated by buds and by eggs 

 according to the season of the year. Dr. Carpenter says that this 

 process of budding must be regarded as a modification of the ordinary 

 nutritious process like the power of reparation so remarkable in all 

 animals, but which exist in greatest degree among the lowest tribes, 

 and especially in the Hydra, which may be cut into pieces, each part 

 becoming a perfect animal. 



The Hydrozoa, then, may be defined shortly as those coelenterata 

 in which the walls of the digestive cavity are not separated from 

 those of the general body cavity, the two coinciding ; and the repro- 

 ductive organs are in the form of external processes of the body, 

 wall buds, in fact, on the one hand, or, of external protuberances, 

 containing either spermatozoa, at the base of the tentacles, or ova, 

 which appear in autumn towards the base of the column, forming a 

 connecting link between gemmation, and ordinary sexual generation. 



The Actinozoa differ from the Hydrozoa in this important respect 

 that they have a space between the digestive cavity and the 

 general body cavity, i.e., the cavity bounded by the outside 

 of the animal, so that it resembles a "vessel suspended in another 

 by an upper turned-over edge, like a double glue-pot, the inner 

 vessel having a hole below, by which it communicates with the cavity 

 of the outer. The great family of Actinian Zoophytes, or Anthozoa, 

 numbers many genera and species. The common sea anemone is 

 the type of all such of them as inhabit the stony corals, and build 

 the coral reefs and atolls of the tropical Pacific, they being essentially 

 the same in structure, but having no sucking disc at the base. The 

 sea anemone has a cylindrical body, attached at one end by a sucker 

 to rocks ; at the other end is also a disc, more or less concave, 

 having a mouth in its centre. This is usually elongated in one di- 

 rection, and at the extremity of the long diameter are folds which 

 are continued into the gastric cavity. It is surrounded by a series 

 of tentacles, which vary considerably in number, but are generally in 



