lxii THE VOYAGE OF H.M.S. CHALLENGER. 



eight thin, broad leaves in the principal radial planes, and run in the umbrella cavity, 

 stretching freely from the subumbrella to the oesophagus ; their longitudinal muscular fibres 

 (" musculi mesenteriales ") pass into the proboscis muscles, at the proximal margin of the 

 mesogonia. Among the Acraspedse the bell muscle of the Cubomedusse usually comports 

 itself like that of the Craspedotge (PI. XXVI. ; System, taf. xxv. xxvi.). The subumbrella 

 of the Tesseronia is on the whole almost cubical ; its coronal muscle consists of four 

 broad quadrangular(often almost quadrate or rectangular) muscular plates, which touch at 

 right angles in the interradii. They are divided by four narrow, interradial longitudinal 

 muscles, running along the four cathammal septa, but are halved in the middle by four 

 broader perradial longitudinal muscles ; the latter pass below upon four " frenula velarii " 

 (PI. XXVI. fig. 8, vf), above upon the mesenteries which run to the four corners of the 

 stomach (PI. XXVI. figs. 2, 3 ; System, taf. xxvi. figs. 2, 3, gm). In the remaining 

 Tesseronia (both in the Stauromedusse and the Peromedusge) the three strong, broad trian- 

 gular deltoid muscles (Pis. XV.-XXII. md) occupy the place of these narrow, band-shaped 

 longitudinal muscles. The deltoid muscles spring with a broad base at the proximal 

 margin of the coronal muscle (mc), and run with converging fibres towards the bottom 

 of the umbrella cavity (fig. F, md). The four interradial deltoid muscles (md) are usually 

 considerably stronger than the four perradial (md) ; the former are inserted at the four 

 interradial cathammal nodes (kn), and often pass out above them as " intergenital 

 muscles " (PI. XX. fig. 8, ms) ; the latter are inserted at the four perradial palatine nodes 

 (gk), and pass from them upon the mesenteries and the corners of the oesophagus. In the 

 Ephyronise or Discomedusse, those parts of the bell muscle appear much less important than 

 in the Tesseronia?, which is accounted for by the retrograde formation of the four perradial 

 pouches, and by the extension of the broad umbrella corona. The eight deltoid muscles 

 (and especially the four interradial) are pretty strongly developed only in a few Canno- 

 stomae (as in Atolla, PL XXIX. fig. 3, and Nauphanta, PI. XXVIII. fig. 12), whilst in 

 most Discomedusse they have undergone retrograde formation. We may therefore regard 

 the four strong pillar muscles of many Semostornse and KhizostomEe as developments of 

 the four perradial deltoid muscles ; they pass at their proximal end into the four perradial 

 proboscis muscles. The system of the marginal muscles (" musculi marginales ") forms 

 the third and distal section of the radial muscular system. Under the term " marginal 

 muscles " we include all the longitudinal or radial muscles which are developed on the 

 umbrella margin outside the circular coronal muscle. They are differentiated in many 

 varied ways. The most important are the muscles of the tentacles and of the marginal 

 lobes. The muscular fibres of the tentacles all run longitudinally in a great variety of 

 arrangements. 



§ 90. Muscular system of the exumbrella. Development of the muscular system 

 appears entirely wanting on the upper or exurnbral surface of the umbrella, when con- 

 trasted with its powerful and universal development on the lower or suburnbral surface. 



