124 THE OCEANIC HYDROZOA. 



centre of this disc there appears to be a small circular foramen, ^^^^ih. of an inch in diameter, 

 and each compartment appears to have a similar but rather smaller foramen terminating 

 a conical prolongation of the compartment. These foramina look so clear and plain that 

 it is difficult to persuade oneself they do not open on the surface. But they do not. 

 They are, in fact, covered over by a considerable thickness of the laminated pneumatocystic 

 substance, which is wholly imperforate. 



Each of the eight radiating ribs probably indicates the position of one of the valleys 

 of the pneumatocyst when it was not larger than the central chamber. Filled up in 

 the adult state by the additions which have been made to the pro.ximal wall, each valley 

 would become more opaque than the intermediate ridge, which must necessarily have 

 less deposit over it, and hence must look like a more solid rib. 



Of the fifty or sixty concentric chambers which surround the central one, those midway 

 between the centre and the circumference are, on the whole, larger than the central or 

 the peripheral ones. 



The pneumatic foramina (/") have an average diameter of about gJoth of an inch. Each 

 is situated at the extremity of an urn- shaped process of the pneumatocyst, which has very 

 delicate walls, and is always seated upon one of the ridges — never in a valley. The urn-shaped 

 process (woodcut J3, p. 126) is about ^i^th of an inch high, and has nearly the same width in 

 the middle, but it narrows to its peduncle below and to its mouth above. The mouth is 

 surrounded by an everted and deflected lip, about 2,'i^th of an inch wide, whose margins are 

 slightly denticulated. 



Every ridge exhibited a pneumatic foramen for each of the ten or twelve outer chambers 

 of the pneumatocyst, except in the case of those ridges which were too short to extend 

 over so many chambers; and all their foramina were, so far as I could observe, open. 

 Beyond this point, however, the foramina occurred far less frequently, and while they were 

 continued nearly to the centre along some radii, stopped far short of it along others, so that 

 in no case have I found more than twenty-four foramina in one radial line ; and more than 

 this, so far as I have been able to ascertain, all the foramina on the central side of the 

 twelfth or thirteenth are closed by the extension of new layers of the substance of the 

 pneumatocyst over them. 



The thin distal wall of the pneumatocyst is somewhat arched and concave under 

 the central chamber, but is otherwise smooth. Eight groups of pneumatic filaments (of 

 which there are four or five in each group) arise from this wall opposite the pneumatic 

 foramina in the proximal wall. 



The pneumatic filaments are simply tubular prolongations of the thin distal wall, 

 which have an average diameter of yc^isth of an inch, and exhibit annular constrictions at 

 intervals. They arise close together, but can hardly be said to have a common stem. 



In the succeeding chambers the distal wall is produced into lamellar folds, which radiate 

 like the ridges of the proximal wall, though they are two or three times more numerous, and 

 become deeper towards the periphery, where they end in margins which are convex 

 downwards and outwards. At their deepest they make up about half the total vertical 

 diameter of the pneumatocyst (woodcut C, p. 126). 



The concentric septa, whicii divide the chambers (/'") of the pneumatocyst from one 

 another, present, in. a vertical radial section, a contour which is convex peripherally, and slopes 



