338 PORPITA PACIFICA. 



there are apparently no stages connecting the two types of granules, even in 

 the transition region between inferior and superior canals, seems to forbid this 

 conclusion. In general appearance as well as in their relation to the cells, they 

 strongly suggest the colored granules which are so prominent in the endoderm 

 of various Medusae, and I believe they are by-products of digestion. The 

 entoderm cells of the inferior canals enclose great numbers of the transparent 

 greenish guanin crystals (Plate 29, fig. 13) so often described for P. umbella. 

 They occur in greatest numbers nearest the lower surface of the centradenia, 

 {. e., furthest from the superior canals. I have likewise observed them in the 

 radial canals of the medusa-buds on the gonophores. It is to be observed that 

 thoy do not occur in the walls of the canals which open into the central gastro- 

 zooid, at least in the neighborhood of the latter; indeed, if they are to be regarded 

 as by-products of digestion, as is no doubt true, their presence would not be 

 expected there. 



With regard to the probable function of the different regions of the centra- 

 denia but little can be said with confidence. The region of the superior canals 

 was long ago suggested as a liver, that of the intermediate and inferior canals 

 a kidney. The canal system of the whole is no doubt digestive, and the inter- 

 pretation of the guanin secreting portion as a kidney is probably correct. How- 

 ever it is doubtful whether there is any region of actual excretion. On the con- 

 trary, in the absence of excretory pores, it should be rather looked on as an organ 

 for the storage of the waste products of metabolism. The interpretation of 

 the superior canals as functioning as a liver in Porpita is supported by little 

 actual evidence. Whether or not it is correct is doubtful. 



Young stages of Porpita pacifica, 1.5-4 ^nyn. in diameter. Although the 

 studies of Woltereck (:04) on the larval stages, and of Bedot ('85b) on 

 the "Rataria" stage of Velella have afforded an excellent survey of the develop- 

 ment of that genus, the early ontogeny of Porpita still remains almost unknown. 

 A. Agassiz ('83) it is true, was able to trace the changes which take place 

 in the canals of the limbus and of the pneumatophore with development, as 

 well as the formation of tentacles of successive generations after what may be 

 known as the " Discalia " stage. But our only knowledge of the early structure 

 of the centradenia is contained in the brief account by Haeckel of the "Dis- 

 conula" larvae, and of the "Discalia" and "Disconalia" stages. Of the former 

 he says ('88b, p. 32) that "in the smallest Disconula-larvae which I observed 

 the centradenia is a small circular, biconvex, lenticular disc; its upper face is 

 in direct contact with the pneumatosaccus, whilst its lower face is separated from 



