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



(figs. 6, 10). The living specimen which I observed at Pola assumed moreover the 

 peculiar attitude represented in Plate VIII. fig. 7. The Medusa lies on its back, extends 

 a portion of the sucking-feet stiffly out round it, and attaches itself to the bottom of the 

 glass, whilst the other portion of the sucking-feet play freely in the water, as if feeling 

 and fishing for prey ; the mouth, therefore, stretches vertically from the opening of the 

 velum, which is contracted like a funnel, and also moves as if groping in different 

 directions. The tentacles, when extended, are almost as long as the radius of the 

 umbrella ; when contracted they are much shorter ; in the centre they are thickened like 

 a spindle, and become thinner at either end. A more minute investigation of the ten- 

 tacles shows that we can distinguish two different forms. The larger number have a 

 sucking-disk, which is pigmented red at the end, and are used for crawling and adhesion 

 by suction ; the smaller numbers are simply pointed at the end, without sucking-disk, 

 and are used as feelers, usually extended round and upwards, and moving like worms 

 (figs. 6-10). 



The velum (fig. 3, vn ; fig. 7) is very thick, broad, and powerful. The plate of its 

 circular muscles is arranged in numerous circular folds projecting from the subumbral 

 surface, which, by interference, produce an iridescence. It is probable that in 

 Pectanthis (as in Pedis and Pectyllis) the velum can be extended so as to entirely 

 close the umbrella cavity like a sphincter. The circular muscles of the subumbrella form 

 projecting circular folds similar to those of the velum, but are arranged in sixteen arcades 

 corresponding to the sixteen exumbral ribs and to the sections of the umbrella margin 

 between every second lobe (fig. 9, wn). The subumbral exoderm is distinguished by 

 scattered nematocysts (fig. 9, wn). The umbrella cavity is divided, as in Pectyllis 

 arctica, into eight separate compartments, or funnel cavities, as the eight broad leaf- 

 shaped mesogonia (fig. 9, ivr) are stretched between the radial canals and the basis 

 of the stomach (see below). 



The central four-lobed oral opening leads into a tube-shaped four-sided stomach, from 

 whose basis eight radial canals (four perradial and four interradial) run out in the 

 bottom of the umbrella cavity. These bear the eight genitalia as sack-shaped evagi- 

 nations in their proximal half, and are united in the umbrella margin by a circular 

 canal which sends out branches into the tentacles. The central oesophagus (fig. 9, gb) 

 hangs down in the middle of the umbrella cavity as a muscular tube of a gold-yellow 

 colour, and prismatic quadrate form. The four interradial lateral surfaces are slightly 

 depressed into a groove between the four rounded perradial bodies. The oesophagus 

 is nearly as long as the radius of the umbrella when extended, much shorter when con- 

 tracted. The oral opening is divided by four shallow grooves into four short blunt oral 

 lobes (al) armed with an accumulation of nemocysts. The mouth can project out of 

 the umbrella cavity through the opening of the velum, and extend externally in the 

 form of a very thin flat octagonal sucking-disk (fig. 3, am). 



