32 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



gastro-vascular system to ectoderm of the surface. The outermost cells of 

 the endodermal lamella make direct connection with the ectodermal 

 cells, without the usual intervention of a layer of gelatine. 



The marginal lamella of Charybdea lies, as the name implies, just on 

 the bell margin where the edge is curving round into the velarium. All 

 around the wh'ole circumference of the bell it is found (in Charybdea) at 

 this same horizontal bend, except in the eight principal radii, where the 

 tentacles and the sensory clubs have brought about modifications. In 

 any place except these a vertical section through the margin will show 

 the marginal lamella connecting the endoderm of the marginal pocket 

 with the ectoderms of the surface, as represented by vim in Fig. 38, which 

 is a vertical section through the sensory niche a little to one side of the 

 perradial axis. 



In the interradii the marginal lamella undergoes modifications due 

 to the fact that the bases of the pedalia are situated a little upwards 

 from the exact margin, and that the lamella follows the outline of the 

 bases. Fig. 1 shows one of the interradial corners of the bell margin 

 looked at directly from the surface, so that the curved outline of the 

 junction of the base of the pedalium with the exumbrella is seen. The 

 trace made by the lamella where it meets the surface ectoderm follows 

 this outline. The lamella is also shown in the vertical section through 

 the interradius (Fig. 5 or 20, vim), where it is seen running from the 

 connecting canals (cc), which joins the two adjacent marginal pockets, 

 upwards and outwards to meet the surface ectoderm. Its course from 

 canal to surface is not in a direct line, but curved with the concavity 

 upwards. Hence, in cross-sections at certain levels through the inter- 

 radial corner it is met more than once and gives rise to appearances that 

 seem at first sight too complicated for it to be just the same structure as 

 the simple marginal lamella described above. That it is the same, and 

 that the complication is only due to the insertion of the pedalia above 

 the margin, can be determined by following through a series of cross- 

 sections, the essential ones of which, as I hope, are given in Figs. 40-43. 

 The levels of these are shown on Fig. 5 by the letters M>, x, y and z, 

 respectively. Fig. 40 shows the lamella cut but once, just below its highest 

 part. The section is above the level of the connecting canal and hence 

 still shows the vertical interradial lamella ivl. Fig. 41, at the next lower 

 level (.E), shows the same portion of the lamella intersected a little nearer 

 the interior, while the junction with the endoderm of the connecting 

 canal is shown still further inside. Fig. 42 is at level y, just through the 



