328 PORPEMA PRUNELLA. 



fig. 2) has been noted by Haeckel ('88b) as has the fact that its structure is 

 less complex than in Porpita. Its apical surface, as seen when stripped of the 

 overlying pneumatocyst and surrounding tentacular zone, bears prominent 

 radial ridges (Plate 27, fig. 12), interlocking with the lower surface of the pneu- 

 matocyst. In one specimen there are seventeen ridges (each indicating the 

 course of one of the superior canals) at the periphery; but of these only the 

 "octoradial liver star" of Haeckel, reaches the apex. In the centre there is a 

 small circular area where no ridges are present, but in this region the union of 

 the eight primary superior canals (Plate 27, fig. 13) can be easily traced. The 

 figure thus formed more nearly resembles that in Velella (Bedot, '85b, pi. 9, fig. 

 2) than in Porpita (Plate 28, fig. 16). Between the canals the tracheae, passing 

 down into the underlying region (Plate 27, fig. 6), can be seen. 



The main mass of the centradenia, as in Velella and Porpita, consists of a 

 dense parenchyma of ectoderm cells and cnidoblasts in every stage of develop- 

 ment, through which the complex network of canals can be traced. The latter 

 arise in the apical region as downward branches from the superior canals, exactly 

 as they do in Porpita (p. 336) ; basally they connect with the central zooid. 

 Haeckel ('88b, p. 62) states that even in the adult {P. medusa) there are only 

 eight canals opening into the latter. Judging from analogy with the condition 

 in Porpita (p. 338) this statement is probably true of very young stages. But in 

 the large specimens collected by the "Albatross", in addition to these eight 

 primary openings, there are about thirty others, which lie near the periphery 

 of the base of the zooid (Plate 28, fig. 11). These are easily traced on sections 

 (Plate 27, fig. 7, 8, 9) ; but in alcoholic material the lumen of the canals is often 

 entirely masked. The eight primary canals do not run directly to the eight 

 primary superior canals of the centradenia as Haeckel supposed, but connect 

 with the general canal-net of that organ. Concerning the tracheae I need merely 

 confirm Haeckel's statement that they ramify throughout the centradenia, 

 and frequently run into the walls of gonozooids and of the central gastrozooid. 



The entodermic walls of the centradenial canals in Porpema are less complex 

 histologically than they are in Porpita. There is some distinction between the 

 canal walls in the upper "liver" ' and in the lower "kidney" portions; it is 

 less marked than in the latter genus. In the "liver" portion the cells are small, 

 and pale-staining, containing granules; in the "kidney" they are more columnar, 

 and enclose masses of large pigmented spherules lying both in the cell bodies 

 and in the intercellular spaces, while among them are to be seen larger, deeper- 



' The use of the terms " hver " and " kidney " in this connection is purely conventional. 



