There can be little doubt that new nematocysts are constantly formed during life to replace those that have been discharged and lost. Each nematocyst is developed within the cell-substance of a cnidoblast which is derived from the undifferentiated interstitial cell-groups. During this process the cnidoblast does not necessarily remain stationary, but may wander some considerable distance from its place of origin.[284] This habit of migration of the cnidoblast renders it difficult to determine whether the ectoderm alone, or both ectoderm and endoderm, can give rise to nematocysts. In the majority of Coelenterates the nematocysts are confined to the ectoderm, but in many Anthozoa, Scyphozoa, and Siphonophora they are found in tissues that are certainly or probably endodermic in origin. It has not been definitely proved in any case that the cnidoblast cells that form these nematocysts have originally been formed in the endoderm, and it is possible that they are always derived from ectoderm cells which migrate into the endoderm. It is probably true that all Coelenterata have nematocysts, and that, in the few cases in which it has been stated that they are absent (e.g. _Sarcophytum_), they have been overlooked. It cannot, however, be definitely stated that similar structures do not occur in other animals. The nematocysts of the Mollusc _Aeolis_ are not the product of its own tissues, but are introduced {249}into the body with its food.[285] The nematocysts that occur in the Infusorian _Epistylis umbellaria_ and in the Dinoflagellate _Polykrikos_ (p. 131) require reinvestigation, but if it should prove that they are the product of the Protozoa they cannot be regarded as strictly homologous with those of Coelenterata. In many of the Turbellaria, however, and in some of the Nemertine worms, nematocysts occur in the epidermis which appear to be undoubtedly the products of these animals. The Coelenterata are divided into three classes:— 1. HYDROZOA.—Without stomodaeum and mesenteries. Sexual cells discharged directly to the exterior. 2. SCYPHOZOA.—Without stomodaeum and mesenteries. Sexual cells discharged into the coelenteric cavity. 3. ANTHOZOA = ACTINOZOA.—With stomodaeum and mesenteries. Sexual cells discharged into the coelenteric cavity. The full meaning of the brief statements concerning the structure of the three classes given above cannot be explained until the general anatomy of the classes has been described. It may be stated, however, in this place that many authors believe that structures corresponding with the stomodaeum and mesenteries of Anthozoa do occur in the Scyphozoa, which they therefore include in the class Anthozoa. Among the more familiar animals included in the class Hydrozoa may be mentioned the fresh-water polyp _Hydra_, the Hydroid zoophytes, many of the smaller Medusae or jelly-fish, the Portuguese Man-of-war (_Physalia_), and a few of the corals.