REPORT ON THE DEEP-SEA MEDUSAE. 

 Survey of the two muscular systems of the sul mmbrella. 



lix 



§ 88. Circular muscles of the subumbrella (" myosystema circulare "). The circular 

 muscular system of the subumbrella is developed quite analogously in the two sections 

 of the Medusae class, and consists of transversely-striated muscles, running in horizontal 

 transverse planes (perpendicularly to the principal axis). This system is divided into 

 three different sections, — the proximal circular muscle of the oesophagus and the oral 

 organs (" musculus orbicularis "), middle coronal muscle (" musculus coronaris "), and the 

 distal circular muscle of the velum (" musculus velaris "). The circular oral muscle 

 (" musculus orbicularis," mo) forms the proximal part of the circular system, and is gene- 

 rally the weakest of its three sections and also the most irregularly developed in the 

 different groups. It only attains any considerable development in such Medusae as are 

 distinguished by a strong, movable oesophagus, or by large oral lobes or folded oral arms ; 

 for example, among the Craspedotse, on the folded oral lobes of many Anthomedusse and 

 Leptomedusae, on the raised lips, capable of extension into a large sucking-disk, of many 

 Trachomedusae (PI. III. fig. 2 ; PI. V. figs. 3, 4 ; PI. VII., fig. 3, am) ; and on the very 

 contractile and extensible oesophagus of many Narcomedusae (Pis. IX.-XIV.) ; among the 

 Acraspedse on the oral lobes of the Stauroniedusae and Cuboniedusae (Pis. XV., XVII. , 

 XXVI.) ; on the buccal pouches of the Peromedusae (PI. XX. figs. 9-11), and on the oral 

 arms of many Discomedusas (Pis. XXX.-XXXIL). The second and middle section of the 

 circular muscular system, the large coronal muscle (" musculus coronaris," mc) is much 

 more important. It must be regarded originally in all Medusae as the most important 

 swimming muscle, and in most of them, it occupies the greater part of the subumbrella, 

 from the distal margin of the orbicular muscle (or in others of the bell-muscle) to the proxi- 



