PORPEMA PRUNELLA. 327 



show that his specimens of P. prunella and P. globosa both exhibit the condition 

 seen in the present series. No doubt he was deceived by surface views, mistaking 

 the lobes of the central chamber for separate compartments. This error is a 

 natural one, on account of the location of the eight inner stigmata which surround 

 the central stigma and which, in surface views, appear to open into the eight 

 lobes of the latter. Their true location, however, is shown in longitudinal sec- 

 tions (Plate 26, fig. 2, Plate 27, fig. 5), on which it is seen that the central chamber 

 broadens basally, and that the first circular chamber partly covers it distally. 

 The eight "primary" stigmata in reality open into the first circular chamber. 

 I shall have occasion to return to this subject in the account of Porpita (p. 335). 

 In a specimen 5 mm. in diameter there are eight rows of stigmata (Plate 26, 

 fig. 3) ; but their number in each row varies at different ages with the increase 

 in the number of concentric chambers. 



The vaulted form of the pneumatocyst and the ridges in its lower surface 

 which interlock with the radial ridges of the centradenia have been described 

 in detail by Haeckel ('88b). The tentacles (Plate 25, fig. 6) and gonozooids 

 (Plate 25, fig. 8, 9) resemble the corresponding organs in Porpita. In the long 

 and the two short rows of tentacular nematocyst knobs there are the same 

 number, 19-24, in adult tentacles. The number of these is a systematic char- 

 acter of some importance in Porpita, and it may prove to be significant in the 

 present genus. 



Canal system of the tentacular zone. Although Haeckel's account of the 

 general organization of Porpema leaves little to be desired, his description of 

 the vascular system requires some correction. The canals which proceed down- 

 ward from the junction of limbus and pneumatophore break up in the tentacular 

 zone into an irregular network (Plate 26, fig. 5, C. T.) which connects with the 

 hollow entodermic cores of the tentacles. This canal-net is represented in 

 the "Challenger" report (Haeckel, '88b, pi. 48, fig. 3) as continuing downward 

 between the centradenia and the bases of the gonozooids, and finally opening 

 into the central zooid. As a matter of fact, however, the canals of the tentacular 

 zone open into the superior canals of the centradenia, exactly as they do in 

 Porpita. The gonozooids open directly into the centradenial canals; thus 

 neither tentacles nor gonozooids are in communication with the central zooid 

 except through the medium of the centradenia. Haeckel's description of P. 

 globosa ('88b, p. 62) suggests that he himself observed this fact, though his figure 

 is not clear. 



Centradenia. The conical form of this structure in Porpema (Plate 26, 



