REPORT ON THE DEEP-SEA MEDUSAE. xxxiii 



by a network of fine threads of protoplasm, traversing the cavity of the cell (PI. I. fig. 7). 

 The nucleus sometimes lies in the middle of the axial cord, sometimes at the one end. 

 These chordal cells belong to the largest cells of the body of the Medusa ; they are 

 often visible to the naked eye, being about 1 mm. broad (p. 38). They sometimes also 

 form a special chordal ring at the umbrella margin (PI. VIII. fig. 8, y). In the short 

 thick tentacles, the chordal cells of the tentacle axis seem sometimes disposed in layers 

 (PI. IV. figs. 5-8, yt ; PI. VI. figs. 12-15, clt). The base of the axis is usually still continu- 

 ously connected with the endoderm of the coronal canal ; more rarely it becomes com- 

 pletely separated from it, and, therefore, mesodermal (Pis. IV., VI., XII., &c). In the 

 gelatinous tissue the small cells retreat entirely against the powerful intercellular sub- 

 stance ; the reverse is the case in the chordal tissue. 



§ 51. Muscular tissue (" tela muscularis "). The muscles of all Medusas consist of fine 

 muscular fibiillas laid parallel, which are connected somewhere with a small lump of proto- 

 plasm containing nuclei, and must, therefore, be regarded as filamental processes of mus- 

 cular cells. The fibrillas are usually very long and thin, sometimes cylindrical, sometimes 

 flattened like a ribbon ; in most longitudinal or radial muscles the fibrillas are smooth, not 

 striated, but they are more or less plainly striated in the transversal or circular muscles. 

 Both the smooth and the striated muscular cells originate for the most part from the 

 ectoderm. In isolated spots, however, both kinds are also formed from the endoderm (as 

 for example at the oesophagus and the oral arms). With regard to the relation of the 

 muscular cells to their original place of formation, the epithelium, we distinguish two 

 principal forms of the muscular tissue ; epithelial muscular cells and mesodermal muscular 

 cells ; the latter still lie in the true epithelial layer of the upper surface or immediately 

 below it, whdst the latter have become completely separated from it and form an inde- 

 pendent, though thin, mesodermal layer. 



§ 52. Epithelial muscular cells ("myoblasti epitheliales," "tela muscularis epitheli- 

 alis "). The majority of the Medusas muscles, that is in the section of the Craspedotas, 

 are composed of smooth or striated fibrillar, whose muscular cells do not form an inde- 

 pendent mesodermal layer, but either belong to the endodermal epithelium itself or to a 

 subepithelial layer lying immediately below it. The fibrillas of these " epithelial muscular 

 cells " or " neuro-muscular cells " therefore lie immediately under the epithelium from 

 which they proceed, and on the supporting plates on which they are borne. They are 

 usually placed in a parallel layer beside each other, or arranged in several layers one above 

 the other in such a way that they form flat leaves or lamellas. By further develop- 

 ment of the muscles, these " muscular leaves " become arranged in folds, whilst the sup- 

 porting laruellas bearing them forms corresponding composite folds by local thickening, 

 as, e.g., in the larger hollow tentacles of the Geryonidas, and of the Cyaneidas, &c. ; in 

 the velum of some Craspedotse (PI. VI. figs. 13, 14). In the smaller and lower 

 Medusas, the broad coronal muscle forms a simple, smooth, band-shaped plate on 



(ZOOL. CHALL. EXP. — PART XII. — 1881.) M e 



