114 .MORPHOLOGY. 



network. Scliultze has recognised the close resemblance between this structure and that of 

 certain forms of the so-called connective tissue. 



Though I have failed in my attempts to detect structure in the gelatinous portion of the 

 umbrella of those hydroid medusse which I have examined, I do not desire on that account to 

 insist on its absence. The comparatively small size and soft consistence of most of the hydroid 

 medusae, and the consequent extreme difficulty of obtaining thin slices fitted for microscopical 

 observation, and freed from the confusing presence of the epithelium and fibrillated layers, 

 throw much greater obstacles in the way of a satisfactory examination of the imibrella of the 

 ordinary hydroid medusae than what we meet with in the larger and more easily manipulated 

 umbrella of the Geryonidce and Stef/anophtlialniafa. 



Lying immediately on the concave surface of the gelatinous substance of the umbrella, an 

 inner epithelial layer (a, c) may under circumstances favorable for observation be demonstrated. 

 It consists of a single layer of cells, and corresponds to the epithelium of the convex siuface, but 

 is much more difficult to detect. 



The concavity of the umbrella is lined by a sac (a, (/) which lies immediately upon the inner 

 epithelium layer, and consists of a distinctly fibrillated membrane. The fibres composing it take, 

 when at rest, a circular course parallel to the margin of the umbrella, and ai-e usually in close 

 contact with one another, though occasionally they become separated at intervals, so as to leave 

 numerous fusiform spaces between them (c). They are very fine, measuring about the ^o^oq of 

 an inch in diameter, and under a high power of the microscope each fibril appears resolved into 

 a single series of corpuscles, a structure which under the action of acetic acid becomes distinct (d). 

 At the margin of the umbrella the fibrillated layer leaves the gelatinous bell and is inflected 

 inwards over the codonostome, so as to constitute the perforated diaphragm or velum. 



The fibrillae of the umbrella and velum, which are thus much more minute than those which 

 have been described in the trophosome, present a marked resemblance to the ultimate fibrillae 

 of striated muscle ; but, instead of being united into fibres, they are spread out into a broad 

 membrane.^ That the fibrillated layer forms a true contractile tissue, conferring on the medusa 

 those active natatory powers which constitute one of its most striking characters, there cannot 

 be any doubt. 



' Busk, in a paper full of excellent observations on the structure of some hydroid medusfe (' Trans. 

 Mic. Soc. Lend.,' vol. iii, p. 14), describes the muscular fibres in the umbrella of Turris neylecia and 

 in that of a Thaumantias-V\ke medusa as " distinctly marked with transverse strise." 



In the swimming-bells of the Siphonophora the fibrillated tissue is very well developed. In a 

 small species of Diphya, captured abundantly on the Irish coast, it was easy in very fresh specimens to 

 get a good view of the contractile fibres which are largely developed in the swimming-bell of this 

 Siphonophore. Besides the circular fibres, a longitudinal set seems also to be present. The circular fibres 

 are flattened, and marked with close transverse striaj, which are rendered particularly evident by the 

 application of acetic acid, which also brings out in the walls of the fibre distinct but distant nuclei 

 with contained nucleoli. 



According to Haeckel, the fibrillated layers of the umbrella and velum of the Gerijonidcc, as well 

 as that which invests the stiff tentacles which always e.xist in the young state of these medusfe, consist 

 of very distinctly striated fibres, while smooth muscle-fibres occur in the walls of the manubrium. 

 He also refers certain fibres and nucleated fusiform cells, which he has detected in the extensile marginal 

 tentacles of these medusee, to the group of smooth muscle-fibre. 



