CASSIOPEA XAMACHANA. '_> | 7 



Development of the (ratstnc Pom-hex. While the final result is the same, the method 

 of development of the gastric pouches differs entirely in our larva from the process as 

 described by Goette. The free-swimming larva is somewhat flattened laterally (Fig. 4), 

 and the long and short diameters are in the planes of the perradii. But there is no 

 evidence that the gastric pouches in the long diameter are formed any earlier than those in 

 the short diameter, and the ectoderm plays no part in their development. The formation 

 of the gastric pouches is usually described as a process of evagination ; but in this case, at 

 least in the earliest stages, the delineation of the pouches seems to be due rather to the 

 ingrowth of the septa. 



These appear at the time of the formation of the first tentacles (Fig. 8) as four 

 minute vertical folds of the endoderm (sep, Fig. 43) , placed equidistantly in the angle 

 formed by the peristomal fold (compare Fig. 42) . The mesogloeal portion of the septum 

 is at first very thin, and in the four-tentacle stage (Figs. 8 and 43) is no higher than the 

 thickness of the endodermal layer of cells. The septal muscles do not penetrate the septa 

 at this stage. While well-developed below, they can be traced upward only to within 

 35 or 41 /z. * from the base of the septa. 



In the eight-tentacle stage (Fig. 13) the septum is still very small, and the septal 

 mesogloea near the margin of the peristome is very thin, hardly thicker than a cell wall. 

 But at the central margin it has increased in thickness, and now the septal muscle may 

 be traced from the stem upward through this thickened portion of the septal mesogloea to 

 the peristomal ectoderm (sm, Figs. 44 and 47). It is impossible to determine whether 

 the new portion of the septal muscles is formed by growth upward of the muscles already 

 present in the stem, or whether it is the result of a proliferation of the peristomal ecto- 

 derm which may grow downward and fuse with the older portion of the septal muscles. 



The four septa are now complete, and divide the digestive cavity into a large 

 central stomach, extending into the stem, and four shallow marginal gastric pouches. 

 The gastric pouches expand with the growth of the peristomal disc, so that the ecto- 

 derm and endoderm remain in close contact at the margin ; while the central edges 

 of the septa retain their original relative position. Thus the gastric pouches and the septa 

 become deeper as the larva increases in size. This is evident in the sixteen-tentacle stage 

 (Figs. 15 and 45 to 49) . It will be shown later that the relation of the interradial ten- 

 tacles to the septa is variable. In this stage, however, the distal part of the septal meso- 

 gloea has begun to disappear, so that immediately under the interradial tentacles the 

 endoderm of adjacent pouches is fused (Figs. 45, 46, and 47) . In the fully developed 

 scyphistoma this area of fusion is perforated, so that there is a communication between 

 adjacent pouches, forming the " Ringsinus" of German authors. In the specimen with 



1 35 or 41 (i = about one seventh of the length of the larva. 



