MUSCULAR SYSTEM. 3 
Substance —When we examine a naked-eyed Medusz by polarized light, we see that at 
least two distinct tissues enter into its composition; these are equally distinguishable by the 
naked eye, if the creature be sufficiently large, when we see the one presenting the aspect of 
a transparent, and almost always colourless gelatinous membrane, the other a translucent, 
and as if granular, substance. The former constitutes the mass of the body; the latter 
forms the margin of the mouth, the edge of the umbrella, and the tentacula. The first 
is immoveable and uncontractile, elastic, but not extensile; the second is highly contractile 
and active. They are both composed of cells, those forming the active tissue differing in being 
nucleated cells of the fibrous order, and intermingled with granular corpuscles. The former 
are covered with a fine amorphous smooth epidermis, beneath which, in the higher forms and 
in the so-called Oceania cruciata—possibly in most species—are cells containing a spiral 
thread. Such cells are also present on the surface of the tentacula in many species, exactly 
as in the hydroid polypes. Will described the cells beneath the epidermis of Geryonia as 
round, transparent, and lobed. A ciliated epithelium has been observed by Will in the inner 
surface of the lip-ring in Geryonia, also in the tentacular canals. It lines (as I have seen) 
the gastro-vascular canals in Zhawmantias, and probably in all the genera. Several of the 
higher Medusze (conspicuously those of the genus Cyan@a) have the power of stinging 
severely. The power resides in the skin, and, especially in some of the appendages of the 
sub-umbrella, appears to be always connected with the second or granular tissue. Wagner 
has attributed it to the filiferous vesicles, which, im some species if not in all, have the 
power of projecting the contained thread with its barbed extremity, even as the hydroid 
polypes and the Actineze do. But as many Medusz and Actinez provided with these curious 
organs do not sting, such explanation is doubtful. I have never found any of the naked-eyed 
species to sting. 
Muscular System.—The motor tissue in these Medusz is of the simplest kind, and 
consists, in most cases, simply of bands of the granular substance just described. In certain 
genera, especially in Zwrris, the motor bands exhibit a distinct fibrous arrangement (Plate III, 
f. 1, e, and f. 2, 7), and Professor R. Wagner has stated that distinct muscular fibres with 
transverse striae are present in the “ Oceania ecruciata” (a Thaumantias?) of the 
Mediterranean.* Will has observed a few longitudinal fibres in the motor ring of a 
Geryonia. In the higher Medusz the muscular system is much more developed, especially in 
Rhizostoma, the movements of which may be shown experimentally to depend on the mus- 
cular bands lining the sub-umbrella. I have paralysed one side of a Rhizostoma Aldrovandi, 
whose disk measured more than a foot across, by removing with a scalpel the bands of that 
half, whilst the other side contracted and expanded as usual, though with more rapidity, as if 
the animal was alarmed or suffering. All the Medusz when irritated become much more 
rapid in their movements, and contract and expand their disks or bodies in a hurried and 
irregular manner, as if endeavouring to escape from their persecutors. In the naked-eyed 
species, the muscular system usually consists of a marginal motor ring, the tissue of which is 
continuous with the tissue of the marginal tentacula; concentric rings of motor tissue forming 
the walls of the tentacles themselves, and a ring of similar tissue forming the margin of the 
* Ueber den Bau der Pelagia noctiluca und der organization der Medusen. 1841. 
