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



or subglobular (in tbe Porpalidae), sometimes more flatly expanded or discoidal (in tbe 

 Porpitellidse). Its vertical main axis is in the first case little shorter or nearly as long 

 as the horizontal or equatorial axis ; whereas in the second case it is much shorter, 

 only one-half or one-third as long, or even less. The free prominentmargin, or the 

 glandiferous limbus umbrella}, marks the boundary between the exumbrella (upper or 

 pneumatophorous half) and the subumbrella (lower or siphonophorous half). 



Exumbrella. — The upper, apical or proximal, part of the discoidal trunk, which 

 corresponds to the exumbrella of the Medusas, and physiologically represents the 

 nectosome, is in all Porpitidae circular and composed of the pneumatocyst, filled with 

 gas, and the surrounding pneumatophore. This latter is composed of two parallel 

 membranes, separated by a network of anastomosing radial canals — the outer thicker 

 membrane armed with thread-cells and pierced by the stigmata, is the pneumatocodon ; 

 the inner thinner membrane, immediately including the pneumatocyst, is the pneumato- 

 saccus or the invaginated exoderm. Both membranes are connected by numerous branched 

 radial septa, and the cavities between these, radiating from the centre of the disc, are 

 the radial canals of the exumbrella (pallial vessels) ; they open at the margin of the 

 disc into the large circular canal of the linib. 



The surface of the exumbrella is in many Porpitidae smooth, in others more or less 

 papillate, owing to conical protuberances of the pneumatocyst. Often an elegant multi- 

 radiate pigment-star is visible, produced by a dark pigment in the wall of the pallial 

 vessels. The central part of the exumbrella in all Porpitidae is pierced by a central 

 stigma, and a surrounding corona of eight stigmata placed in the walls of the eight 

 surrounding radial chambers. The other stigmata of the exumbrella, in larger species 

 several hundreds, in smaller only few, are sometimes disposed in regular radial rows, at 

 other times more irregularly scattered. 



Pneumatocyst. — The chitinous polythalamous float filled with gas, which is called 

 pneumatocyst (formerly "inner shell"), is in the Porpitidae always regularly circular, 

 corresponding to the surrounding pneumatosaccus (or the invaginated exumbrella) from 

 which it is secreted. Consequently its general form in the subfamily Porpalidae is highly 

 vaulted, campanulate or cap-shaped (Porpalia, Porpema, Pis. XL VII. , XL VIII. ), whereas 

 in the flatly expanded Porpitellidse it is discoidal, even or slightly vaulted (Porpitella 

 and Porpita, Pis. XLV. and XLVL). The pneumatocyst is always composed of two 

 little distant and nearly parallel lamellae of structureless chitin, which are connected 

 by numerous concentric annular septa. The latter divide the float into numerous 

 concentric ring-chambers, and these open on the upper or proximal face by stigmata, 

 on the lower or distal face by tracheae. 



The central disc of the pneumatocyst has in the Porpitidae the same structure as in 

 the Discalidae ; it is composed of a spherical or subglobular central chamber (with a 

 central stigma above) and of eight equal triangular radial chambers, each of which bears 



