Order =PENNATULACEA= (/Free, phosphorescent; colonies, with horny axis; polyps in lateral branches/) ^Pennatula^ [pg142] CLASS =ACTINOZOA= SEA-ANEMONES, REEF-CORALS, SEA-FANS, SEA-WHIPS, AND SEA-PENS The animals of this class are divided into two subclasses, based on the following anatomical differences: In the first subdivision, which includes the sea-anemones and the reef-building corals, the polyps have numerous simple, hollow tentacles and radial partitions of the same number, both being some multiple of six. The polyps of a colony are all alike, and the hard matter they secrete is carbonate of lime. In the second subdivision, the /Alcyonaria/,—or halcyonoids, as they are commonly called,—the tentacles and radial partitions are always eight in number, and the tentacles have small symmetrical appendages or branches. The polyps of this group are often accompanied by small zoöids having no tentacles. The hard secretions are horny and elastic, as in sea-fans, sea-whips, and sea-pens, or they are extremely hard, as in /Corallina rubrum/. SUBCLASS =ZOANTHARIA= ("/Animal-flowers/") ORDER =ACTINIARIA= SEA-ANEMONES These curious and, at times, beautiful creatures are polyps, and are constructed on the radial system. They have a cylindrical body with a broad base and rows of hollow tentacles around the upper disk. They have a central sac, and a space between the outer and inner sacs which is divided vertically by walls called /mesenteries/, as explained on page 114. Each tentacle, however many there [pg143] may be, has a corresponding chamber. The inner sac has a mouth, like a slit, at the top of the disk, opening to the outside, and it also has openings into the chambered spaces which surround it. These chambers also open into the hollow tentacles; thus a continuous circulation throughout the whole animal is established. The food taken in at the mouth is digested in the inner sac, passes through all the chambers of the cavity as nutritive fluid, and is then expelled at the mouth again. The inner sac, or gullet, has longitudinal grooves; two of these are broad and deep, and correspond to the corners of the mouth. These are called /siphonoglyphs/. The sea-anemone is soft and contractile, and belongs to the only order of this class which does not secrete a skeleton. It has two sets of muscles, one of which extends from the base to the summit of the body and is placed on the dividing partitions. Sea-anemones are classified by the arrangement of the septa and the manner in which the muscles are placed upon them. The other set of muscles is arranged around the circumference of the column or body. Each tentacle is furnished with similar sets of muscles. The animal is sensitive, and at the least alarm contracts its body by means of these muscles, and quickly transforms itself from a beautiful, flower-like form into a shapeless, unattractive, inconspicuous mass.