REPORT ON THE SIPHONOPHOR^E. 69 



alternating, and these more than all the other lamellae The centrifugal dichotomous 

 ramification of the larnelke increases gradually towards the periphery, and new peripheral 

 folds are interposed between the centrifugal folds, so that their number at the margin 

 of the disc is much greater than that of the simple radial ridges of the upper surface. 

 The crest of the folds is elegantly denticulate, and from it arise two opposite radial rows 

 of tracheae. The tracheae or aeriferous tubules in this Porpita are exceedingly numerous, 

 amounting to many thousands. The majority of them are short and end half way up the 

 centradenia. A smaller number pierce the latter and the subjacent support, and project 

 into the exodermal wall of the central siphon and the surrounding gonostyles. They 

 terminate here in the basal half of the latter, inside the exodermal epithelium (fig. 8, pt). 



The circular concentric ring-chambers of the pneumatocyst communicate one with 

 another by radial apertures or " pneumothyrae," oblicpie elliptical openings, which are 

 placed in the radial sulci between the folds of the lower face. 



The central chamber possesses, as usual, a central stigma in its upper face, as does also 

 each of the surrounding eight radial chambers. From the basal part of each of the latter 

 arises a bunch of three or four short tracheae. In the basal sulci between these are eight 

 openings, which lead into the first ring-chamber. 



Centradenia (fig. 1, uh, meridional section; fig. 3, inferior face; figs. 6-8, un, parts 

 of sections). — The large central gland (or the so-called "liver") is a biconvex lenticular 

 disc, the horizontal diameter of which is four times as great as its vertical axis ; the latter 

 measures in a specimen of 20 mm. diameter (umbrella) 3 mm., the former 12 mm. Its 

 superior face has a regular radiate appearance ; innumerable radial folds (thirty-two of 

 which are stronger) fitting into the radial grooves which lie between the lamellar pouches 

 of the inferior face of 'the pneumatocyst. The height of these folds increases from the 

 centre towards the periphery. The circular margin meets the line which marks the 

 boundary between the gonostylar and tentacular zones. 



The inferior face of the centradenia is less convex than the superior, and is divided 

 into a white central disc (kidney) and a peripheral brown radiate zone (liver). The 

 former covers the roof of the central siphon (sa), and surrounds it like a broad, white 

 girdle (fig. 3, un) ; the peripheral part of the brown liver (fig. 3, sh) surrounds again the 

 former as a dark radiate ring. The gonostyles arise as well from the former as from the 

 latter. 



The meridional or vertical section of the centradenia (fig. 1) demonstrates that its 

 solid exodermal parenchyma is traversed by a dense network of innumerable canals, the 

 greater part of which are darkly coloured by a brown (hepatic) pigment. This is want- 

 ing in a part of the superficial canals, and in the inferior white central disc (" kidney"), 

 which contains masses of guanin-crystals. Innumerable tracheae terminate between 

 the canal branches ; their distal openings being surrounded by (air-secreting ?) exoderm 

 cells (compare the explanation of figs. 6-8). 



