336 PORPITA PACIFICA. 



this statement, though essentially correct, needs emendation.' The mass of 

 canals known as the "kidney" occupies the entire lower face of the central gland 

 outward from the central zooid so far as the inner margin of the tentacular zone, 

 and it is only by removing the latter that the outer ends of the superior ("liver") 

 canals become visible from below (Plate 29, fig. 1). 



The radial canals, which run downward from the junction of limbus with 

 disc, open into the outer ends of the superior canals or sinuses (Plate 29, fig. 1). 

 The latter in their turn connect with the tentacles by means of a complex net 

 of thin-walled canals, which form a plexus underlying the entire tentacular zone 

 (Plate 29, fig. 1). So far as I have been able to learn from a study of numerous 

 sections, only the youngest, most peripheral tentacles ever connect directly with 

 the radial canals. This is evidently only temporary, for with growth the point 

 of demarcation between the two categories of canals moves further and further 

 distally, successively overlapping the new tentacles shortly after their formation. 



The difference between the structure of the tentacular zone in Porpita and 

 in Porpema (p. 327) is no doubt correlated with the differences in the shape of 

 the corm in the two genera. There are a few gonozooids scattered among the 

 tentacles and these connect either directly or via the tentacular plexus with the 

 superior canals (Plate 29, fig. 1). Haeckel is thus correct in stating that some 

 of the gonozooids are connected with the "kidney" others with the "liver." 

 Central to the tentacular zone the superior canals rest upon the mass of ectoderm 

 cells and cnidoblasts traversed by the inferior canals, which form the great bulk 

 of the centradenia. The inferior canals arise, as noted by Bedot ('85a) as down- 

 ward branches from the superior canals (Plate 29, fig. 7) and form a complex 

 network. At the inferior surface of the centradenia, immediately in contact 

 with the supporting layer ("lamelle anhiste" of Bedot, '84) they give rise to an 

 extremely close reticulum of small horizontal canals (Plate 29, fig. 6) to which 

 alone the term "inferior" was applied to Bedot. It is, however, in their arrange- 

 ment alone, not in their structure, that the latter differ from their parent canals. 



According to Haeckel ('88b, p. 70) there are sixteen main canals, arising 

 from the central gastrozooid and running peripherally, "Along the subumbrella 

 toward the peripheral edge of the limb. . . .they give off innumerable branches; 

 of these the ascending ones enter into the centradenia, the descending vessels 



' To avoid the use of the terms "hver" and "kidney" until more accurate linowledge of the func- 

 tions of the various regions of the centradenia is gained, the following nomenclature, somewhat 

 emended from that of Bedot ('84) is employed. The canals forming the ridges on the upper face of 

 the organ are known as "superior," those below forming the greater mass of the organ ("kidney"), 

 as "inferior". The canals running downward from the level of the limbus toward the tentacular zone, 

 following Haeckel, as radial." 



