Chap. VII. OBELIA COMMISSURALIS. 



319 



the tentacles are thrown more strongly upward, so as to be nearly parallel with 

 the axis of the proboscis, or, in the systolic act, they are, as well as the disk, 

 strongly curved downwards, and form a deep bell-shaped cavity about the proboscis. 

 The proportions of the disk are like those of an old-fashioned bull's-eye watch-glass, 

 thickest in the centre, and thinning out to an edge. The centre is occupied by 

 a four-sided digestive cavity {Fig. 12, h% from which a simple, trumpet-shaped 

 quadrate proboscis {p) hangs down, to a depth which, in fall extension, equals 

 the semidiameter of the disk. The four corners of the proboscis correspond to 

 the four corners of the digestive cavity. From the digestive cavity (/i*) four 

 radiating canals (/>) extend from its four corners to near the edge of the disk, 

 where they connect with a circular canal (/) which passes through the Avhole 

 circuit of the margin. Within these canals a constantly circulating current is 

 kept up by means of large vibratile cilia {Figs. 20 and 21, /, /). Of the sixteen 

 tentacles, there is one opposite the termination of each radiating canal, and three, 

 arranged at equal distances, in every quarter segment between the canals. 



Seen from above, every tentacle seems to taper gradually from the base to 

 the apex {Fig. 20), but, upon looking deeper, its actinal end appears enlarged into 

 a broad swelling (;'), which, however, when observed more closely and from below 

 {Fig. 21, J'), proves to be a two-fold lobe of the edge (a) of the disk, embracing 

 the base of the tentacle. The outer wall {Fig. 12, «i) of every tentacle is con- 

 tinuous with the outer wall of the disk («), and the inner wall {b'^) of the same 

 is a prolongation of the middle wall {h) of the disk. The base of each tentacle 

 has a broad and rounded prolongation {Figs. 12 and 20, (i), which projects toward 

 the centre of the disk, and across the actinal side of the circular canal. The 

 eight eyes {Fig. 12, a) are affixed to the actinal side of the base of the eight 

 tentacles which stand, one on each side of the radiating canals. Each eye is a 

 globular body {Fig. 21, a), containing, at its centre, another globular body («') 

 about one quarter its diameter, and possessing highly refracting properties. The 

 eye stands out from the surfoce, and, in profile {Fig. 18% «), is a very conspicuous 

 object. The transverse veil {Fig. 12, v), which borders the margin of the disk, 

 is about one eighth the diameter of the latter, more or less wavy at the edge, 

 and very transparent and thin. The proboscidal actinostome {Fig. 12, j^) is double- 

 walled, as in other Hydroids ; the outer wall (;j) is thin and continuous with the 

 innermost or lining wall (/, and Figs. 20 and 21, g, g') of the disk, and the inner 

 wall {p) is very thick and continuous with the middle wall in which the radiating 

 and circular canals are hollowed out {Figs. 20 and 21, /, f^). The arrangement 

 of the lasso-cells on the tentacles is peculiar; in a view from the abtictinal side 

 {Fig. 20), we see a single row along the middle, and a double row on each edge; 

 whereas, on the actinal side {Fig. 21) there is no central row. Each lasso-cell is 



