220 ROBERT PAYNE BIGELOW ON 



contractile, and may be spoken of as a marginal lobe of the peristome (Figs. 19 and 

 20). In the specimen shown in Fig. 21 we see the first indication of stabilization. The 

 upper, expanded part of the calyx is separated from a conical, lower portion by a slight 

 groove. The marginal lobes have become semicircular in outline, and a slight elevation is 

 noticeable on the aboral side of each rhopalial tentacle immediately external to the mass 

 of concretions. The epithelium at this point is pigmented, and forms the first rudiment 

 of the eye (oc. Fig. 22) . Fig. 23 illustrates a more advanced stage, where the proximal 

 part of the tentacle is beginning to take on its final shape, and is separated by a pro- 

 nounced bend from the distal portion, which is still functional as a tentacle. 



We come finally to a stage in which, while the long distal part of the tentacle retains 

 its characteristic structure and remains completely functional, the short proximal part 

 has become completely differentiated into a rhopalium. Fig. 54 is from a longitudinal 

 section of such a tentacle. The rhopalial part has assumed nearly its final shape. The 

 differentiation of its ectoderm into sensory epithelium, eye-spot, and layer of nerve fibres, 

 is complete. It has a lumen that extends outward to the solid chorda-like endoderm of the 

 distal part of the tentacle, and opens toward the centre into a gastric pocket. The endo- 

 dermal lining of the lumen is a columnar epithelium, the more distal cells being deeper 

 and containing the concretions. Compare Fig. 54 with Fig. 53, which, being interradial, 

 was certainly destined to be a rhopalial tentacle. 



The growth of the marginal lobes, which were semicircular at the stage of Fig. 21, 

 has continued, and each lobe has now produced two secondary ones, one on each side of 

 the rhopalial tentacle. These are connected by a slight ridge that crosses the base of the 

 tentacle on its aboral side (h. Fig. 54) . The secondary lobes are the rhopalial lobes of 

 the margin of the umbrella (Flugellappen of German authors) , and the connecting ridge 

 is the hood (Deckplatte) that covers the rhopalium. These marginal structures may be 

 seen in Fig. 24, and this brings us to another stage in the development of the rhopalium, 

 the absorption of the distal part of the tentacle. 



In the strobila shown in Fig. 24, the rhopalial tentacles have a very different 

 appearance from what we have seen before. They are shorter than the other tentacles, 

 and are much swollen at a point just beyond the eye-spot. The distal portion is begin- 

 ning to degenerate. This process, when once begun, proceeds rapidly. During the few 

 hours that were spent in making this drawing, the rhopalial tentacles were reduced in 

 length nearly one half. The eye-spots and concretions were conspicuous, and in each of 

 the former there was a slight cup-shaped depression. This is the earliest stage in which I 

 observed slight medusa-like movements of the ephyra disc. The tentacle at this stage is 

 in a process of degeneration for about fifteen hundredths of a millimeter outward from the 

 ocellus. In this area of degeneration ( t. Fig. 55) the endodermal cells are broken down, 



