PORPITA PACIFICA. 335 



25-29 in the long rows, and from 11-14 in each of the short rows in the largest 

 specimens of P. pacifica. 



In general structure the pneumatocyst agrees with Haeckel's account, 

 except for the important fact that there are no radial chambers surrounding 

 the central chamber. As Bedot ('85a) has pointed out for P. umbella, the sup- 

 posed radial chambers are nothing more than radial lobes of the central compart- 

 ment, just such as occur in Porpema (p. 326). The supposed septa separating 

 them, which are very noticeable on surface views, are nothing more than furrows 

 on the upper surface of the central chamber (Plate 28, fig. 7). The structure 

 of this region was accurately described by Huxley ('59, p. 123), who also observed 

 that the stigmata near the centre of the disc were closed. 



Ceniradenia. Adult. The courses of the tracheae have been so thoroughly 

 worked out by Kolliker ('53), Huxley ('59), A. Agassiz ('83) and others, that 

 I will merely note that my slides show frequent instances of the penetration 

 of these organs into the walls of the central gastrozooid and of the gonozooids. 



The general structure of the centradenia is now fairly well known, but we 

 possess little more information as to its finer details in Porpita than is contained 

 in the brief account (without figures) by Bedot ('85a). It is therefore fortunate 

 that the present series is sufficiently well preserved to allow the study of serial 

 sections. In general form the centradenia of P. pacifica agrees closely with 

 that of P. umhella, as it does in its finer anatomy, so far as that of the latter is 

 known. The superior surface (Plate 28, fig. 3) of this organ, in P. pacifica 

 as in P. umhella, is thrown into numerous ridges interlocking with the plications 

 of the floor of the pneumatocyst. Haeckel ('88b, p. 69) calls these ridges "innum- 

 erable radial folds (thirty-two of which are stronger)." In point of fact, there 

 are only about sixteen at the centre, while the number increases distally until in 

 a specimen 55 mm. in diameter upwards of two hundred could be counted near 

 the margin. At the centre there is a small circular, or sometimes octagonal, 

 area, corresponding in size to the superposed central chamber of the pneuma- 

 tocyst, where the surface is smooth. In this region the canals, which further 

 distally occupy the radial ridges, unite in an irregular network (Plate 28, fig. 16) 

 the exact form of which varies in different specimens. In the space between 

 the canals, groups of tracheae pass downwards into the substance of the 

 centradenia. 



Haeckel says of the inferior surface of the central gland that it is divisible 

 into a white central disc ("kidney") and a peripheral brown radiate zone 

 ("liver"). Leaving out of account the colors, the present specimens show that 



