PORPITA PACIFICA. 



339 



the central siphon by the gastrobasal plate ("plancher" of Bedot). The entire 

 mass of this solid disc is composed solely of exodermal cells and cnidoblasts; it 

 is not traversed by any canals. The only canals of the centradenia are the 

 eight simple radial canals which run upon its upper face; they arise from the 

 eight ostia of the basigaster, embrace the surface of the lenticular centradenia 

 like eight meridian lines, and unite in the centre of its uj^per face, forming a 

 typical octoradial 'liver star'." A similar condition is described by him ('88b, 

 p. 48) for the "Discaha" stage. 



Although no figures of the "Disconula" are given as evidence for this 

 account, the structure as he outlines it agrees so closely with the oldest Velella 

 larva figured by Woltereck (:04, taf. 19, fig. 18) that we can safelj- assume that 

 it is correct. 



A somewhat more advanced, but still very i:)rimitive condition is to be 

 seen in specimens of P. pacifica 1.5-2 mm. in diameter, with eight gonozooids, 

 and twenty-four tentacles 

 (Fig. B). A surface view 

 of the centradenia of one of 

 these shows that the eight 

 primary superior canals, 

 uniting at the centre as ob- 

 served by Haeckel, fork 

 di- or trichotomously until 

 there are twenty canals or 

 ridges at the margin, and, 

 as can be worked out on 

 serial sections, the number 

 of ostia opening into the 

 central zooid has increased 

 to twenty-eight. These are 



arranged in two series, an inner row of eight, connecting by means of short 

 vertical canals with the eight primary superior canals, and an outer series of 

 twenty, close to the margin of the zooid. The outer ostia connect with the outer 

 ends of the canals (fig. B) by solid cell strands without lumen, a fact which, of 

 course, indicates that the communication between them and the canal system of 

 the central gland is more recent than it is in the case of the eight ostia of the 

 inner series, which no doubt correspond to the eight gastric ostia of Haeckel's 

 "Disconula" larva. In our specimen each inner ostiinn connects with a canal 



-P 



Fig. B. Section of young Porpita pacifica 2 mm. in diameter. 

 Ch., circular chamber. C.S.. superior canal. C',C'., descend- 

 ing canals of the younger and older series. C. T., tentacular 

 canal. Go., gonozooid. Ga., central gastrozooid. L, linibus. 

 T', T-, older and younger tentacles. St, central stigma. 



