REPORT ON THE DEEP-SEA MEDUSAE. lxiii 



Closer investigation, however, shows that muscles are also developed here in some places, 

 though but feebly, at least in some groups of Medusas (perhaps in all ?), and then into both 

 transverse and longitudinal cords of fibre. The principal exumbral circular muscles are 

 the feeble zonal muscles (" musculi zonares," mz) which are found in some Craspedotse 

 above the umbrella margin, in others in the circular stricture between the umbrella and 

 the apical process {e.g., Catablema, System, taf. iv. fig. 4). They are more strorjgly 

 developed in some Acraspedas as, for example, in the visible zonal muscle of the Pero- 

 medusse which divides the smooth umbrella cone from the pedal zone of the umbrella 

 corona and sends out zigzag processes between its pedalia (PI. XXIII. fig. 34, mz ; PI. 

 XXIV. fig. 2, mz). Exumbral radial muscles are found in some groups (especially in the 

 Trachomedusas and Narcomedusse), developed on the umbrella margin into peronial 

 muscles (" musculi peroniales," PI. XIII. fig. 7, ml; PI. XIV. fig. 12, ml). Other, but 

 feebler, longitudinal muscles appear in both sections here and there on the umbrella apex 

 and on other places of the convex outer umbrella surface. Among the Craspedotse, four 

 perradial and four interradial longitudinal muscles are found in the peripheric (and some- 

 times also in the central part) in some Anthoniedusse ; and among the Acraspedse, in the 

 Cubomedusse. The longitudinal muscular bands of the tseniola and the strong peduncle 

 muscles of the Stauromedusse, also belong to this system (Pis. XVI., XVII. figs. 13, 

 14, m). 



§ 91. Umbrella cavity or swimming cavity (" antrum, caverna umbralis," h). The 

 umbrella cavity of the Medusas is as characteristic for this class of urticating animals as 

 the umbrella itself ; it is enclosed above by the lower concave surface of the umbrella 

 (" subumbrella"), whilst it opens freely below through the aperture of the umbrella cavity 

 (" apertura antri "). The subumbral umbrella cavity is more or less vaulted, according as 

 the umbrella is more umbrella shaped or more conical ; its vaulted roof, which is fined by 

 the ectoderm of the subumbrella, is, however, always flatter than the outer surface of the 

 umbrella which is covered by the ectoderm of the exumbrella, for the gelatinous wall of 

 peripheric umbrella corona is always thinner than the central umbrella cone. As by each 

 contraction of the swimming Medusas, the vaulting of the umbrella cavity becomes higher, 

 its opening narrower, and water is ejected through the opening, whilst by each dilatation 

 of the umbrella fresh water enters the flattened and widened umbrella cavity, the latter 

 may be regarded physiologically both as a " swimming cavity " and a " respiratory 

 cavity." The ectodermal epithelium of the subumbrella, which fines the umbrella cavity, 

 is probably adapted for respiratory functions. The opening of the umbrella cavity 

 (" apertura antri ") is simple and surrounded by the corona of lobes in the Acraspedse, 

 whilst in the Craspedotse it is narrowed by the velum, which projects freely inwards like 

 a diaphragm, from the umbrella margin. In some Craspedotse the velum is so broad, 

 that it is probably capable of completely closing, for a while, the opening of the umbrella 

 cavity, as in the Pectyllidas (Pis. III.-VIIL), The central axial space of the subumbrella 



