Before passing on to the study of the Infusoria, a few words may be offered on the _Noctiluca_, a genus of animals usually referred to the class ACALEPHÆ. One species only of this genus has been described, which occurs occasionally on the English coast in prodigious numbers. It is a small creature, scarcely the hundredth part of an inch in diameter, according to Mr. Huxley (Fig. 27, _Noctiluca miliaris_). It was discovered by M. Surriray, in 1810, who describes it as a spherical gelatinous mass, scarcely bigger than a pin's head, with a long filiform tentacular appendage, a mouth, an oesophagus, one or many stomachs, and branching ovaries--thus exhibiting a certain complexity of organization. De Blainville took the same view, and placed it among the _Diphydæ_. Van Beneden and Doyère, on the other hand, deny its relation to the _Acalephæ_, conceiving its organization to be much more simple: they place it with the _Rhizopoda_. Quatrefages adopts the same view, denying the existence of a true mouth or intestinal canal: he considers the so-called stomachs as simple "vacuales," similar to those observed in the Rhizopoda and Infusoria. Mr. Huxley, describing it in the "Journal of Microscopical Science" (vol. iii.), says it has nearly the form of a peach, a filiform tentacle, equal in length to the diameter of the body, occupying the place where the stalk of the peach might be, which depends from it, and exhibits slow wavy motions when the creature is in full activity. "I have even seen a _noctiluca_," he adds, "appear to push against obstacles with this tentacle." [Illustration: Fig. 27. Noctiluca miliaris. Magnified.] "The body," he continues, "is composed of a structureless and somewhat dense external membrane, which is continued on to the tentacle. Beneath this is a layer of granules, or rather of gelatinous membrane, through whose substance minute granules are scattered without any very definite arrangement; from hence arises a network of very delicate fibrils, whose meshes are not more than one three-hundredth part of an inch in diameter, which gradually pass internally--the reticulation becoming more and more open--into coarser fibres, taking a convergent direction towards the stomach and nucleus. All these fibres and fibrils are covered with minute granules, which are usually larger towards the centre." Mr. Huxley is inclined to think, from all he has observed, that the animal has a definite alimentary cavity, and that this cavity has an excretory aperture distinct from the mouth.