12 THE VOYAGE OF H.M.s. CHALLENGER, 



same thickness throughout its whole extent (fig. 3). The exumbrella or the outer 

 convex surface of the umbrella is divided by sixteen rim-like radial ribs whose distal 

 ends project lobe-like on the margin of the umbrella, into sixteen deepened radial area? or 

 grooves (PL III. figs. 1,2); eight of the sixteen radial ribs correspond with the under- 

 lying genitalia and mesogonia (four perradial and four interradial). The eight principal 

 ribs unite above in the centre of the exumbrella into an eight-rayed star, while the eight 

 remaining adradial ribs which alternate with these do not reach the top of the umbrella. 

 Each two ribs are united at the distal end by a two-limbed clamp ; whose intercostal centre 

 turns inwards and downwards (figs. 1, 2). This arching inwards forms the boundary 

 In 'tween each two of the sixteen flat marginal lobes, and lies in the ideal prolongation of 

 the sixteen intercostal radial grooves of the exumbrella (" sulci exumbrales "). These form 

 the deepest part of the concave grooves between each two ribs. They are only sharply 

 defined in the middle zone of the exumbrella, and obliterated below and above. A very 

 delicate radial rib runs in the exumbrella between each groove and each rib (fig. 1). 



The umbrella margin (" margo umbralis," figs. 1, 2, 4, 10, &c.) is considerably thickened, 

 and even at a superficial view appears divided, more or less clearly, into sixteen flat, 

 slightly convex, projecting lobes, each of whose central points forms the distal end of a 

 radial exumbral rib. The entire margin of the umbrella is continuously beset with nume- 

 rous tentacles (above a thousand), which are placed one over the other in several rows, 

 and are transformed into short-stalked sucking-cups. Isolated auditory clubs (8 or 16 ?) 

 are placed between them. A very broad velum projects inwards from the ring of cnida? 

 of the umbrella margin. 



The number of marginal tentacles amounts to 1000 or 1200 ; all bear a terminal 

 sucking-cup. Only a small number of them appear in the form of longer or shorter 

 filaments ; in the greater number the filaments part of the tentacle is so much reduced, 

 and the terminal sucking-cup so strongly developed, that they appear as short-stalked 

 or even unstalked suckers (" acetabula"). The distribution of these appendages is very 

 regular and peculiar. Each of the sixteen marginal lobes bears a principal group, com- 

 posed of three larger sucking-cups and three triangular sucking-plates alternating 

 with them, and in each sucking-plate we can distinguish sixteen to twenty sucking- 

 cups of different sizes (comp. figs. 1, 2, 4, 10). Sixteen very large sucking-cup- 

 of the first size are the keystones of the arches, at the end of the sixteen exumbral 

 ribs (fig. 7, xa; fig. 10, A). Thirty-two sucking-cups of the second size lie some- 

 what depressed between them (fig. 8, xb ; fig. 10, B). Between these two series placed 

 still deeper and with longer stalks there are forty-eight suckiug-cups of the third 

 size (fig. 5, xc ; fig. 10, C). Finally, under these, closely crowded together in a 

 triangular group, are numerous smaller sucking-cups with longer stalks, their number 

 increasing towards the insertion of the velum, while their size diminishes (fig. 6, xd). 

 Each of these so-formed " sucking-plates" has a great resemblance to the sucking-plates at 



