334 PORPITA PACIFICA. 



special mention on account of their specific importance. Chief among these 

 are the tubercles on the superior surface of the disc. Haeckel ('88b) observed 

 and figured these; but he gave them no systematic weight. In the large speci- 

 mens collected by the "Albatross" (Plate 28, fig. 1) they are very prominent, 

 easily visible to the naked eye, largest and most crowded near the centre of the 

 disc. They have no connection with the stigmata and this fact is important, 

 because in P. umhella the latter organs, especially in large specimens, are sur- 

 rounded by slight collars, causing a roughness, noted by A. Agassiz ('83). The 

 tubercles in P. pacijica are conical thickenings of the chitinous layer of the 

 pneumatocodon; and the canals of the pneumatophore often pass over them 

 (Plate 28, fig. 4). Although the tubercles are extremely prominent, the radial 

 plications of the surface of the disc are so slight as to be hardly noticeable except 

 near the margin. In P. umhella, on the contrary, the ridges are rather promi- 

 nent. The presence of tubercles in Huxley's ('59) specimens seems to me suffi- 

 cient reason for including them in P. pacifica. 



A second feature which proves to be of systematic importance is the arrange- 

 ment of the stigmata. In the adults of P. umhella, as has long been known, 

 these organs are extremely numerous in the marginal region of the disc, although 

 in large specimens the stigmata in the central region become closed (Plate 28, 

 fig. 13). In P. pacijica (Plate 28, fig. 2) the stigmata are very much less numer- 

 ous. There are no open ones for a distance from the centre of almost 2 the 

 diameter of the disc; and even beyond this point very few open into each cir- 

 cular chamber of the pneumatocyst. In a small specimen (1.5 mm.) the stig- 

 mata in the central region are still open, as Haeckel has described them ('88b, 

 " Porpitella pectanthis"). 



A third specific distinction, though of minor importance, is that in large 

 specimens of P. pacifica the limbus is relatively much narrower than it is in 

 P. U7nbella of about the same size (Plate 28, fig. 2, 13). Further points of dif- 

 ference are found in the canals of the limbus, and in the tentacles. In P. umhella 

 the former permanently retain their general radial arrangement (Plate 28, fig. 

 14), though with much branching and irregularity. In P. pacifica, however, the 

 canals form an irregular network in adults (Plate 28, fig. 5) though in small 

 specimens they are of the same type as in P. umhella. Finally the tentacles, 

 at least in large specimens, form an excellent specific character, for whereas 

 in P. umhella the stalked tentacular nematocyst clusters are not known to sur- 

 pass 9-12 in the long, and 6-8 in the two short rows (A. Agassiz, '83, Chun, '97b), 

 a fact I have been able to substantiate on numerous specimens, there are from 



