PEOF. G. J. ALLMAN ON LIMNOCODIUM YICTOEIA. 133 



the path travelled over by the tentacle as its insertion moved in 

 the course of metamorphosis from the margin of the umbrella to 

 a point at some distance above it ; while in Limnocodium the 

 ridges are direct continuations of the tentacles, whose structure 

 they retain. They suggest a comparison with the root-like con- 

 tinuation of the tentacles, which are plunged into the substance 

 of the umbrella in the Narcomedusae and in certain Trachome- 

 dusae. Just before reaching the margin they become narrower, 

 and are ultimately inserted on the summits of the sinuses into 

 which the thickened margin is thrown. The intrant angle between 

 the free portion of the tentacle and the umbrella is rounded off 

 by a frenum-like extension (fig. l,f) of the outer epithelium o£ 

 the umbrella. 



I could find no indication of a cavity in the tentacles, a feature 

 in which they resemble the solid tentacles of the Narcomedusse 

 and Trachomedusse. Instead, however, of possessing the peculiar 

 axis composed of large cylindrical or disk-shaped cells laid one on 

 the other like coins in a rouleau, which is so prevailing a character- 

 istic of the tentacles in these orders, the axis is here formed of an 

 irregular tissue of polygonal cells. This pith-like axis is sur- 

 rounded by a membranous tube (which probably consists of the 

 " Stiitzlamelle " with muscular fibres), and this, again, by a layer o£ 

 flattened membraneless cells, whose confluent walls form a con- 

 tinuous naked protoplasmic stratum, which is raised into nume- 

 rous small conical elevations arranged somewhat spirally round 

 the tentacle from its base to its apex. In each of these little 

 protoplasmic tubercles are imbedded three or four very minute 

 fusiform thread-cells, the distal ends of which may be often seen 

 projecting beyond the summit of the tubercle. 



A further affinity of Limnocodium may be traced in the direc- 

 tion of Obelia, as shown not only by the tentacles being in both 

 destitute of a cavity, but by the fact of their not being free in 

 their entire extent, for in Obelia the basal end is plunged, in 

 the form of a root-like continuation, into the substance of the 

 umbrella. Though Obelia is in systematic descriptions included 

 among the Leptomedusaa, I have elsewhere pointed out indica- 

 tions of affinity between it and the Narcomedusal form Cunina*. 

 Its affinity with the Trachomedusas is still closer ; but from both 

 groups, as well, indeed, as from the Leptomedusse, a marked point 

 of divergence is found in the suppression of a velum. 



Notwithstanding the absence of a cavity, the tentacles oiLimno- 

 * British Association Eeports for 1867. 



