MUSCULAR SYSTEM. 3 



Substance. — When we examine a naked-eyed Medusae 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 Thauniantias, and probably in all the genera. Several of the 

 higher Medusae (conspicuously those of the genus Cyanaa) 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, in 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 Actineae do. But as many Medusae and Actineae 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 Medusae is of the simplest kind, and 

 consists, in most cases, simply of bands of the granular substance just described. In certain 

 genera, especially in Turris, the motor bands exhibit a distinct fibrous arrangement (Plate III, 

 f. 1, e, and f. 2, i), and Professor R. Wagner has stated that distinct muscular fibres with 

 transverse striae are present in the " Oceania cruciata" (a Thaumantids f) of the 

 Mediterranean.* Will has observed a few longitudinal fibres in the motor ring of a 

 Geryonia. In the higher Medusae 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 Rhi%ostoma 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 Medusae 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 noctUuca und der organization der Medusen. 1841. 



