F. S. CONANT ON THE CUBOMEDUS^E. 35 



addition the first appearances (as the series is traced downwards) of the 

 arches of the lamella over the two lateral tentacles (cf), which are 

 inserted a little lower down than the middle one of the group. As con- 

 cerns these lateral tentacles, the relations of the vascular lamella at this 

 level are the same as that in the level of Fig. 40 for Charybdea. 



It has been stated more than once already that the vascular lamella 

 of the sensory niche is a part of the lamella that runs round the margin, 

 and so far the only evidence given has been the strip of endodernial 

 fusion running from the marginal lamella to that of the niche. This 

 strip, however, as lias been described, does not come to the surface and 

 consequently seems at first sight to be a different structure from the 

 lamella of the margin. That it is not, however, I found very prettily 

 shown in a series of sections of one of my youngest Tripedalia. In this 

 the lamella of the niche as it was traced in successive sections downwards, 

 was found not to form a closed ring at the bottom of the niche, but each 

 side was continued directly and separately downwards to the margin, 

 where it passed into the corresponding part of the marginal lamella. A 

 reconstruction of the condition, similar to that of Fig. 44, is given in Fig. 

 4.1, and I think explains itself at a glance. Evidently the vascular 

 lamella? that connect the lamella of the sensory niche with that of the 

 margin at first come to the surface, like the rest of the marginal system, 

 but as the animal grows older come to lie within the gelatine. In this 

 way the condition found in cross-sections just through the margin of my 

 very small Tripedalia, and represented in Fig. 46, becomes that of the 

 adult seen in the corresponding portion of Fig. 29. It is as complete a 

 demonstration as could be required that the lamella of the sensory niche 

 is at first only a loop of the marginal lamella, a conclusion that had been 

 already reached by H. V. Wilson on theoretical considerations, based 

 upon the facts of the adult structure as he found them in Chiropsalmus. 



As Wilson pointed out in his notes, these facts have a close bearing 

 upon the question of the origin of the velarium. Sixteen marginal 

 pockets are found in both Chiropsalmus and Tripedalia, and all of them 

 extend into the velarium. It is not unnatural t6 suppose that these 

 belong to sixteen marginal lobes, and that these lobes have fused together 

 to form the velarium. In the Chirodropus figured by Haeckel (Taf. 

 XXVI) in his "System" gelatinous lobe-like thickenings are shown in 

 the velarium, corresponding to the sixteen marginal pockets. In Tripe- 

 dalia no special gelatinous thickenings are found, but the arrangement 

 of the marginal pockets is the same as that of the t'hirodropidie, and 



