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.
(273). In the Pulmonigrade Acalephae, we have the best illustration of this arrangement: in these, the stomach or digestive cavity is excavated in the centre of the disk, and is supplied with food by a mechanism that differs in different species. In Bhizostoma (fig. 49), which receives its name from the nature of the communication between the stomach and the exterior of the body*, the organ destined to take in nourishment consists of a thick pedicle, composed of eight foliated divisions, which hang from the centre of the disk. Each of these appendages is found to contain ramifying canals, opening at one extremity by numerous minute apertures upon the external surface, whilst at the opposite they are collected into four large trunks communicating with the stomach; as the Rhizostoma, therefore, floats upon the waves, its pendent and root-like pedicle absorbs, by the numerous oscules upon its exterior, such food as may be adapted to its nutrition, finding most probably an ample provision in the microscopic creatures which so abundantly people the waters of the ocean. The materials so absorbed are conveyed through the canals in the interior of the arms into the stomachal cavity, where their solution is effected.
(274). But it is not upon this humble prey that some of the Medusae feed; many are enabled, in spite of their apparent helplessness, to seize and devour animals that might seem to be far too strong and active to fall victims to such assailants: Crustacea, worms, mollusca, and even small fishes are not unfrequently destroyed by them. Incredible as this may seem when we reflect upon the structure of these feeble beings, observation proves that they are fully competent to such enterprises. The long tentacula or filaments with which some are provided, form fishing-lines scarcely less formidable in arresting and entangling prey than those of the Hydra; and, in all probability, the stinging secretion which exudes from the bodies of many species speedily paralyses and kills the animals which fall in their way. The mouth of these Acalephse is a simple aperture leading into the gastric cavity, and sometimes surrounded with tentacula, that probably assist in introducing the food into the stomach.
(275). In Cassiopea Borbonica, the principal agents in procuring nourishment are numerous retractile suckers (fig. 50, a), terminating in small violet - coloured disks, which are dispersed over the fleshy appendages to the under surface of the body; the stem of each of these suckers is tubular, and conveys into the stomach nutritive materials absorbed from animal substances to which they are attached during the process of imbibing food.
(276). The above examples will suffice to give the reader an idea of the most ordinary provisions for obtaining nourishment met with in the Pulmonigrada; we will therefore return to consider the structure of the stomach itself, and of the canals that issue from it and convey the digested nutriment through the system. In Cassiopea Borbonica, which will serve to exemplify the general arrangement of these parts in the whole Order, the stomach (fig. 51, b) is a large cavity placed in the centre of the inferior surface of the disk, and is apparently divided into four compartments by a delicate cruciform membrane arising from its inner walls. Into this receptacle all the materials collected by the absorbing suckers are conveyed through eight large canals, and by the process of digestion become reduced to a yellowish semifluid pulpy matter constituting the pabulum destined to nourish the whole body. Prom the central stomach sixteen large vessels arise (fig. 51, c), which radiate towards the circumference of the disk, dividing and subdividing into numerous small branches that anastomose freely with each other, and ultimately form a perfect plexus of vessels as they reach the margin of the mushroom-shaped body of the creature. The radiating vessels are moreover made to communicate together by means of a circular canal (fig. 51, e) which runs round the entire animal, so that every provision is made for an equable diffusion of the nutritive fluid derived from the stomach through the entire system. Now, if we come physiologically to investigate the nature of this simple apparatus of converging and diverging canals, we cannot but perceive that it unites in itself the functions of the digestive, the circulatory, and the respiratory systems of higher animals: the radiating canals, conveying the nutritive juices from the stomach through the body, correspond in office with the arteries of more perfectly organized classes; and the minute vascular ramifications in which these terminate, situated near the thin margins of the locomotive disk, as obviously perform the part of respiratory organs, inasmuch as the fluids permeating them are continually exposed to the influence of the air contained in the surrounding water, the constant renewal of which is accomplished by the perpetual contractions of the disk itself.
Fig. 50. Cassiopea Borbonica.
(277). The umbrella-like disk of Cyanea aurita, whose anatomy has been most carefully studied by Ehrenberg, is composed of a highly organized gelatinous substance invested by three membranous integuments, the structure of which is by no means so simple as has been generally imagined. The exterior of these tegumentary membranes, covering the convex surface of the disk, consists of a dense tissue made up of hexagonal cells containing a soft whitish substance mixed up with little granules, and presents upon its outer surface innumerable little suckers or agglomerations of granular bodies, which are visible to the naked eye.