REPORT ON THE SIPHONOPHOR^E. 81 



the third on the right posterior quadrant of the pneumatocyst, each at the base of a 

 radial chamber (fig. 8, pe' anterior, pe" posterior stigma). 



Tracheal (fig. 9, pt). — The concave inferior face of the pneumatocyst, which is in 

 close contact with the superior convex face of the centradenia, gives off eight small 

 bunches of tracheas, each composed of two to four. These are simple undivided tubules, 

 which arise from the periphery of the eight radial chambers of the pneumatocyst, and 

 enter into the centradenia, where they end in its exodermal parenchyma. 



Centradenia (figs. 3, 4, uc, in profile ; fig. 5, uc, in vertical transverse section ; figs. 6 

 and 7 in horizontal section, fig. 6 through the inferior, fig. 7 through the superior part). 

 — The central gland, or the so-called " liver," has the form of a flat cone, strongly com- 

 pressed from the two lateral sides, or of a thick vertical triangular lamella, which is 

 somewhat sigmoidal, or slightly bent in an S-form. Its upper face is completely covered 

 by the campanulate pneumatocyst ; its lower face is in contact with the gastrobasal plate 

 of the central siphon (fig. 5, st), and the surrounding corona of sexual siphons. 



The glandular parenchyma of the centradenia is composed in the superior half more 

 of entodermal canals, in the inferior half more of exodermal cells and cnidoblasts. The 

 vascular reticulum of the superior face exhibits a star of eight radial main vessels ; two of 

 these, opposite in the longitudinal axis of the disc, are longer and stronger than the six 

 others, which are disposed symmetrically on both sides of the former, three on the right, 

 three on the left ; the middle of these, or the frontal canal, is the shortest, whilst the two 

 diagonal canals of each side are longer than the latter, shorter than the sagittal canals. 1 

 The numerous smaller vessels, which arise from these eight superior hepatic vessels, form 

 an irregular network, which is rather dense in the superior half of the centradenia, loose 

 in its inferior half ; the axial part of the latter is chiefly composed of exoderm-cells. 



The descending superficial canals of the centradenia pass over at its basal periphery 

 into four different groups of canals : — (l) The numerous radial canals of the free mantle 

 border (or pallial canals, fig. 10, ce) ; (2) the canals of the tentacles (fig. 5, t) ; (3) the 

 canals of the gonostyles (gs) ; (4) the eight innermost canals which open into the base of 

 the central siphon. 



The pallial canals (fig. 10, ce), which run nearly horizontally from the periphery of the 

 base of the liver to the edge of the mantle border, are very numerous, and so densely 

 attached one to another, that their intervals are smaller than their lumen. Their distal 

 ends, which open into the annular elliptical marginal canal (cc), are partly simple, partly 

 forked ; usually a simple and a forked vessel regularly alternating. 



Central Siphon (fig. 2, so ; fig. 3, sa ; fig. 5, sa, in longitudinal section). — The large 

 central polypite is a slender inverted cone, strongly compressed from both sides. Its ellip- 

 tical basal plate is in contact with the basal face of the centradenia, and separated from 

 it by the strong fulcrum of the gastrobasal plate (fig. 5, st). The periphery of this is 



1 Compare the figure of Bedot, 59, pL ix. fig. 1. 



(ZOOL. CHALL. EXP. — PART LXXV1I. — 188-'.) Hbllh 11 



