(267). The ocean, in every climate, swarms with infinite multitudes of animals which, from their minuteness and transparency, are almost as imperceptible to the casual observer as the Infusoria themselves; their existence, indeed, is only indicated by the phosphorescence of some species, which being rendered evident on the slightest agitation, illuminates the entire surface of the sea. All, however, are not equally minute, some growing to a large size; and their forms are familiar to the inhabitants of every beach, upon which, when cast up by the waves, they lie like masses of jelly, melting, as it were, in the sun, incapable of motion, and exhibiting few traces of organization, or indications of that elaborate structure which more careful examination discovers them to possess. Their uncouth appearance has obtained for them various appellations by which they are familiarly known, as Sea-jelly, Sea-blubber, or Jelly-fishes; whilst, from disagreeable sensations produced by handling most of them, they have been called Sea-nettles, Stingers, or Stang-fishes. The faculty of stinging is indeed the most prominent feature in their history; so that their names in almost all languages are derived from this circumstance.

They were known to the older naturalists by the title of Urticce marince; and the word at the head of this chapter, applied by Cuvier to the entire class, and originally used by Aristotle, is of similar import (Hydrozoa Acalephje Cuv 62 a nettle).

(268). There are few subjects which come under the observation of the physiologist more calculated to excite his astonishment than the history of these creatures. If he considers, in the first place, the composition of their bodies, what does he find? An animated mass of sea-water; for such, in an almost literal sense, they are. Let him take an Acaleph, of any size, and lay it in a dry place; it will be found gradually to drain away, leaving nothing behind but a small quantity of transparent cellular matter almost as delicate as a cobweb, which apparently formed all the solid framework of the body, and which, in an animal weighing five or six pounds, will scarcely amount to as many grains; and even if the water which has escaped from this cellulosity be collected and examined, it will be found to differ in no sensible degree from the element in which the creature lived. The conclusion therefore at which he naturally arrives is, that, in the Acalephae, the sea-water collected and deposited in the delicate cells of an almost imperceptible film becomes, in some inscrutable manner, instrumental to the exercise of the extraordinary functions with which these creatures are endowed.

(269). The ACALEPHae have been divided by zoologists into groups distinguished by the nature of their means of progression: in describing, therefore, the organs of locomotion, with which we commence their history, the reader will be made acquainted with the principal modifications of outward form exhibited by various races of these interesting beings.

(270). Pulmonigrada

The most ordinary examples of the Acalephae found in our climate, when examined in their native element, are seen to be composed of a large mushroom-shaped gelatinous disk, from the inferior surface of which various processes are pendent, some serving as tentacula, others for the prehension of food. In Rhizostoma (fig. 49) the central pedicle resembles in structure and function the root of a plant, being destined to absorb nourishment from the water in which the creature lives. The body of one of these Medusae is specifically heavier than the water of the ocean, and would consequently sink but for some effort on the part of the animal. The agent employed to sustain it at the surface, and in some measure to row it from place to place, is the umbrella-shaped expansion or disk, which is seen continually to perform movements of contraction and dilatation, repeated, at regular intervals, about fifteen times in a minute, having some resemblance to the motions of the lungs in respiration, whence the name of the order (pulmo, the lung; gradior, I advance.) By these constant movements of the disk, the Medusa can strike the water with sufficient force to ensure its progression in a certain direction when swimming in smooth water; but of course such efforts are utterly inefficient in stemming the course of the waves, at the mercy of which these animals float.

The tentacular appendages, situated around the margin of the disk in such species as are provided with these organs, are likewise capable of contractile efforts, and may in some slight degree assist as agents of impulsion, although they are destined to the exercise of other functions. The locomotive disk, when cut into, seems perfectly homogeneous in its texture, nor is any fibrous appearance easily recognizable, to which its movements could be attributed; nevertheless in the larger species its inferior surface appears corrugated, as it were, into minute radiating plicse, which seem to contract more energetically than the other portions, and resemble a rudimentary development of muscular fibre.

Rhizostoma Cuvieri.

Fig. 49. Rhizostoma Cuvieri.

(271). In the Acalephae, indeed, the substance of the body is generally entirely soft and gelatinous, emulating, in the delicacy of its texture and perfect translucency, the structure of the vitreous humour of the eye, its entire organization apparently consisting of a transparent aqueous fluid contained in innumerable polyhedral hyaline cells. In some species, however, certain parts of the animal are of semicartilagi-nous tissue; and in a few instances cartilaginous or calcareous lamellae are found imbedded in their substance, which may be compared to a rudimentary polypary or internal skeleton.

(272). Interesting as these creatures may justly be considered when we contemplate the singular beauty of their external configuration and the wonderful design conspicuous in their locomotive organs, a more intimate acquaintance with their habits and economy will be found to disclose many facts not less curious in themselves than important in a physiological point of view. In the higher animals, we are accustomed to find the nutritive apparatus composed of several distinct systems - one set of organs being destined to the prehension of food, another to digestion, a third to the absorption of the nutritious parts of the aliment, a fourth provided for its distribution to every part of the body, and a fifth destined to ensure a constant exposure of the circulating fluid to atmospherical influence; these vital operations are carried on in vessels specially appropriated to each; but in the class of animals of which we are now speaking, we find but a single ramified cavity appropriated to the performance of all these functions, and exhibiting, in the greatest possible simplicity, a rough outline, as it were, of systems afterwards to be more fully developed.