This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(286). It is very probable that the older writers, who speak of a circulation of blood in the Medusae, only alluded to the movements observable in the contents of the intestinal ramifications; it appears, however, from Ehrenberg's observations, that in the Medusae there exist distinct globules, which are of a uniform round shape, enclosed in distinct vessels, wherein a kind of circulation is carried on: these globules he describes as being colourless, spherical, simple, and varying from 1/288th to 1/300th of a line in diameter.
(287). Although the Medusae have always been admitted to possess considerable sensibility, no traces of a nervous system had been detected in their soft and delicate tissues until Ehrenberg pointed out a structure apparently of a nervous character. On carefully examining the eight brown-coloured spots which are disposed at equal distances around the margin of the disk (fig. 48, f, f), he found them to present a very elaborate and remarkable organization. Each of these coloured spots is seen, when accurately observed, to be composed of a little button-like appendage, of an oval or cylindrical shape, attached to the extremity of a slender pedicle, which in turn takes its origin from a kind of vesicle, wherein may be perceived, by means of the microscope, a glandular-looking substance. On the dorsal aspect of each of the pedunculated brown-coloured appendages is situated a distinctly marked round spot of a bright red colour, supposed by Ehrenberg to be an ocular organ, while he considers the glandular-looking substance above mentioned to constitute a nervous ganglion. In addition to this arrangement, he considers that there exist, running all along the margin of the disk, in each of the interspaces between the marginal tentacles, a series of ganglia of a similar character, giving off nerves to the tentacula, whilst other ganglia are to be detected in the tentacular appendages situated in the vicinity of the oesophagus, as well as in the oviferous cavities.
In short, he states the general distribution of the nervous matter in the Medusiform Acalephge to be as follows: - Pour groups of nervous ganglia are situated around the oesophagus in the oviferous cavities close to the ovaria, which are in communication with as many groups of tentaeula. Upon the outer border of the disk, close to the base of the marginal tentacles, is another series of nervous nodules, interrupted at regular intervals by the eight brown-coloured corpuscles. Lastly, there exists a series of isolated ganglionic masses, eight in number, situated at the bases of the supposed ocular organs, to which they give off nervous filaments.
(288). The so-called ocular organs, named by Ehrenberg unhesitatingly "pedunculated eyes," present a very remarkable structure. Each "pedunculated eye " is directed towards the dorsal aspect of the disk, and has, situated beneath its lower surface, a minute sacculus of a yellowish colour, but variable in its shape, wherein is contained a number of solid crystals, clear as water, and which the action of acids proves to be composed of carbonate of lime.
(289). Not only eyes, however, but ears also are conceded by modern naturalists to these favoured occupants of the ocean.
(290). At the base of the marginal tentaeula, or cirri, says Professor Forbes*, there are present, in a great many of these animals, coloured spots or bulbs; and in some species these points are so strongly coloured, that, from this circumstance and their magnitude, they indicate the course of the animal when in motion, appearing like a circle of gems in the water. When these bulbs are examined with the microscope, they are found to contain a small cavity, quite distinct from any coloured matter that may be present; the former is regarded by modern naturalists as an otolithic vesicle, the latter as an ocellus, or eye-spot.
(291). The otolithic vesicle, which, from analogy and its peculiar structure, is considered as an organ of hearing, is a small spherical sac, developed in the midst of the granular substance of the bulb, and containing more or fewer minute vibrating bodies. Will has described the otolithic vessel and its contents, as they are found in Geryonia, as follows: - "The auditory vesicles are seated in the course of the marginal circular vessel, in very uncertain number; usually, however, one at each side of the larger marginal cirri. They are round, measuring 1/40th of a line in diameter, and consist of a tolerably thick membrane; they contain from one to nine, and even more, round globules. If there is only one, it is situated exactly in the centre of the vesicle; but if there are several, they are found lying together, either in two groups, or joined to each other. I have never observed them move. Muriatic acid dissolves them, and causes the vesicle to burst." The existence of similarlyconstructed organs has been recognized in many other species by various observers.
* Monograph on the British Naked-eyed Medusae.
(292). It was discovered by Sars*, that certain forms of naked-eyed Medusas multiply their species by means of gemmation, the buds being produced either from the walls of the peduncle or stomachal proboscis, or from the surface of the ovaries. In both cases the new individuals were not different from, but similar to, their parents; and in one instance, provision seemed to be already made in the newly-formed offshoots for continuing to propagate by the same mode other individuals similar to themselves. Prom a certain part of the body roundish knobs grow forth, which gradually assume the shape of a bell, by opening themselves at the free end, and soon present the form of young Acalephs, being merely attached to the mother by means of a short peduncle, derived from the back of the disk. These develope in themselves all essential organs whilst still attached to the mother, like the buds of a plant, until at length, after a certain time, they separate from the parent and swim about as independent individuals.