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



SubumbrcUa (fig. 1). — The inferior, distal or basal face of the umbrella is divided into 

 four different zones. The central zone, with a diameter of 3 to 4 mm., is occupied by 

 the gastrobasis or the fundus of the large central siphon. The broad middle or gonostylar 

 zone (4 to 6 mm. in breadth) is occupied by the numerous sexual siphons or gonostyles. 

 The third or tentacular zone (2 to 3 mm. broad) bears the numerous tentacles. The 

 fourth or outermost zone (1 to 2 mm. broad) is formed by the inferior face of the free 

 mantle-border. 



Pneumatocyst (fig. 1, fig. 5, pf). — The float, filled with air, is a flat circular disc of 15 

 to 20 mm. diameter. Its thickness increases considerably from the centre towards the 

 margin, owing to the height of numerous radial folds. The form of the pneumatocyst 

 presents a great resemblance to a regular Fungia, the disc being folded radially in a very 

 regular manner. The vertical folds arise in the upper surface in eighty or ninety high 

 radial ridges, with deep valleys between them, whilst branched radial lamellae are pro- 

 minent on the lower face. 



The octoradial structure of the pneumatocyst is very distinct on its upper face, since 

 the eight equidistant primary rays are more prominent than all the others. They arise 

 from the septa of the eight radial chambers which surround the central chamber. The 

 octant between every two primary folds is bisected by a secondary ray, somewhat less 

 prominent, but also stronger than all the others. Sixteen tertiary or adradial ridges 

 arise in the middle between the eight primary and the eight secondary rays. The other 

 rays are developed in the peripheral half of the disc only. The total number of radial 

 ribs is about eighty in a specimen of 1 5 mm. diameter, one hundred and twenty in a larger 

 specimen of 20 mm. The most elevated part of each radial rib bears a radial series of 

 stigmata. The number of concentric annular septa is about fifty in the former, eighty in 

 the latter. These are wider (about twice as broad) in the middle part of the disc than in 

 the central and the peripheral part. 



The thickness of the chitinous wall of the air-chambers is much greater in the upper 

 than in the lower face, and it increases from the periphery of the disc towards the centre. 

 It equals in the central half of the disc the height of the air-chambers. Numerous strata 

 of chitinous substance are here deposited as secondary supports of the disc-wall, and have 

 closed the stigmata (fig. 1 , p 2 ). The conical tubercles of the surface of the disc are pro- 

 duced by stronger deposits at separate places (p 1 ). The stigmata of the air-chambers 

 remain open in the peripheral part of the disc only (_pe); they are urn-shaped, very 

 numerous, and irregularly scattered in radial rows (fig. 2, pe). 



The lower face of the pneumatocyst, which is in close contact with the centradenia, is 

 more strongly folded than the upper face ; the folds are here ramified centrifugally, and 

 form high triangular lamellae, the height and number of which increase from the centre 

 towards the periphery (fig. 1, £> 4 ). Eight primary radial lamellae, arising from the septa 

 of the eight pericentral radial chambers, are more prominent than the eight secondary 



