Before describing the different kinds of Polyps living on our immediate coast, we will say a few words of Polyps in general and of the mode in which the structural plan common to all Radiates is adapted to this particular class. In all Polyps the body consists of a sac divided by vertical partitions (Fig. 1.) into distinct cavities or chambers. These partitions are not, however, all formed at once, but are usually limited to six at first, multiplying indefinitely with the growth of the animal in some kinds, while in others they never increase beyond a certain definite number. In the axis of the sac, thus divided, hangs a smaller one, forming the digestive cavity, and supported for its whole length by the six primary partitions. The other partitions, though they extend more or less inward in proportion to their age, do not unite with the digestive sac, but leave a free space in the centre between their inner edge and the outer wall of the digestive sac. The genital organs are placed on the inner edges of the partitions, thus hanging as it were at the door of the chambers, so that when hatched, the eggs naturally drop into the main cavity of the body, whence they pass into the second smaller sac through an opening in its bottom or digestive cavity, and thence out through the mouth into the water. In the lower Polyps, as in our common Actinia for instance, these organs occur on all the radiating partitions, while among the higher ones, the Halcyonoids for example, they are found only on a limited number. This limitation in the repetition of identical parts is always found to be connected with structural superiority. [Illustration: Fig. 1. Transverse section of an Actinia. (_Agassiz_.)] The upper margin of the body is fringed by hollow tentacles, each of which opens into one of the chambers. All parts of the animal thus communicate with each other, whatever is introduced at the mouth circulating through the whole structure, passing first into the digestive cavity, thence through the opening in the bottom into the main chambered cavity, where it enters freely into all the chambers, and from the chambers into the tentacles. The rejected portions of the food, after the process of digestion is completed, return by the same road and are thrown out at the mouth.