GENERAL SKETCH OF THE POLYPS.
Fig. 1. Transverse section of an Actinia. (Agassiz.)
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.
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.
These general features exist in all Polyps, and whether they lead an independent life as the Actinia, or are combined in communities, like most of the corals and the Halcyonoids; whether the tentacles are many or few; whether the partitions extend to a greater or less height in the body; whether they contain limestone deposit, as in the corals, or remain soft throughout life as the sea-anemone,—the above description applies to them all, while the minor differences, either in the tentacles or in the form, size, color, and texture of the body, are simply modifications of this structure, introducing an infinite variety into the class, and breaking it up into the lesser groups designated as orders, families, genera, and species. Let us now look at some of the divisions thus established.
The class of Polyps is divided into three orders,—the Halcyonoids, the Madreporians, and the Actinoids. Of the lowest among these orders, the Actinoid Polyps, our Actinia or sea-anemone is a good example. They remain soft through life, having a great number of partitions and consequently a great number of tentacles, since there is a tentacle corresponding to every chamber. Indeed, in this order the multiplication of tentacles and partitions is indefinite, increasing during the whole life of the animal with its growth; while we shall see that in some of the higher orders the constancy and limitation in the number of these parts is an indication of superiority, being accompanied by a more marked individualization of the different functions.