INTRODUCTION—CLASSIFICATION—HYDROZOA—ELEUTHEROBLASTEA—MILLEPORINA—GYMNOBLASTEA—CALYPTOBLASTEA—GRAPTOLITOIDEA—STYLASTERINA
The great division of the animal kingdom called Coelenterata was constituted in 1847 by E. Leuckart for those animals which are commonly known as polyps and jelly-fishes. Cuvier had previously included these forms in his division Radiata or Zoophyta, when they were associated with the Starfishes, Brittle-stars, and the other Echinodermata.
The splitting up of the Cuvierian division was rendered necessary by the progress of anatomical discovery, for whereas the Echinodermata possess an alimentary canal distinct from the other cavities of the body, in the polyps and jelly-fishes there is only one cavity to serve the purposes of digestion and the circulation of fluids. The name Coelenterata (κοῖλος = hollow, ἔντερον = the alimentary canal) was therefore introduced, and it may be taken to signify the important anatomical feature that the body-cavity (or coelom) and the cavity of the alimentary canal (or enteron) of these animals are not separate and distinct as they are in Echinoderms and most other animals.
Many Coelenterata have a pronounced radial symmetry, the body being star-like, with the organs arranged symmetrically on lines radiating from a common centre. In this respect they have a superficial resemblance to many of the Echinodermata, which are also radially symmetrical in the adult stage. But it cannot be insisted upon too strongly that this superficial resemblance of the Coelenterata and Echinodermata has no genetic significance. The radial symmetry has been acquired in the two divisions along different lines of descent, and has no further significance than the adaptation of different animals to somewhat similar conditions of life. It is not only in the animals formerly classed by Cuvier as Radiata, but in sedentary worms, Polyzoa, Brachiopoda, and even Cephalopoda among the Mollusca, that we find a radial arrangement of some of the organs. It is interesting in this connexion to note that the word "polyp," so frequently applied to the individual Coelenterate animal or zooid, was originally introduced on a fancied resemblance of a Hydra to a small Cuttle-fish (Fr. Poulpe, Lat. Polypus).
The body of the Coelenterate, then, consists of a body-wall enclosing a single cavity ("coelenteron"). The body-wall consists of an inner and an outer layer of cells, originally called by Allman the "endoderm" and "ectoderm" respectively. Between the two layers there is a substance chemically allied to mucin and usually of a jelly-like consistency, for which the convenient term "mesogloea," introduced by G. C. Bourne, is used (Fig. 125).
The mesogloea may be very thin and inconspicuous, as it is in Hydra and many other sedentary forms, or it may become very thick, as in the jelly-fishes and some of the sedentary Alcyonaria. When it is very thick it is penetrated by wandering isolated cells from the ectoderm or endoderm, by strings of cells or by cell-lined canals; but even when it is cellular it must not be confounded with the third germinal layer or mesoblast which characterises the higher groups of animals, from which it differs essentially in origin and other characters. The Coelenterata are two-layered animals (Diploblastica), in contrast to the Metazoa with three layers of cells (Triploblastica). The growth of the mesogloea in many Coelenterata leads to modifications of the shape of the coelenteric cavity in various directions. In the Anthozoa, for example, the growth of vertical bands of mesogloea covered by endoderm divides the peripheral parts of the cavity into a series of intermesenterial compartments in open communication with the axial part of the cavity; and in the jelly-fishes the growth of the mesogloea reduces the cavity of the outer regions of the disc to a series of vessel-like canals.
Another character, of great importance, possessed by all Coelenterata is the "nematocyst" or "thread-cell" (Fig. 124). This is an organ produced within the body of a cell called the "cnidoblast," and it consists of a vesicular wall or capsule, surrounding a cavity filled with fluid containing a long and usually spirally coiled thread continuous with the wall of the vesicle. When the nematocyst is fully developed and receives a stimulus of a certain character, the thread is shot out with great velocity and causes a sting on any part of an animal that is sufficiently delicate to be wounded by it.
The morphology and physiology of the nematocysts are subjects of very great difficulty and complication, and cannot be discussed in these pages. It may, however, be said that by some authorities the cnidoblast is supposed to be an extremely modified form of mucous or gland cell, and that the discharge of the nematocyst is subject to the control of a primitive nervous system that is continuous through the body of the zooid.
There is a considerable range of structure in the nematocysts of the Coelenterata. In Alcyonium and in many other Alcyonaria they are very small (in Alcyonium the nematocyst is 0.0075 mm. in length previous to discharge), and when discharged exhibit a simple oval capsule with a plain thread attached to it. In Hydra (Fig. 124) there are at least two kinds of nematocysts, and in the larger kind (0.02 mm. in length previous to discharge) the base of the thread is beset with a series of recurved hooks, which during the act of discharge probably assist in making a wound in the organism attacked for the injection of the irritant fluid, and possibly hold the structure in position while the thread is being discharged. In the large kind of nematocyst of Millepora and of Cerianthus there is a band of spirally arranged but very minute thorns in the middle of the thread, but none at the base. In some of the Siphonophora the undischarged nematocysts reach their maximum size, nearly 0.05 mm. in length.
