94 MEMOIES OF THE NATIONAL ACADEMY OF SCIENCES. 



Stage " F," 5 Days Old. 

 (Figures 22-30.) 



Althougli " F " is separated from tlie stage last described by a considerable space of time, the 

 changes in the larva which have been brought about are easy to follow. 



The larva is considerably larger tlian iu " E," and has reached its full development. From 

 this time on the larval organ gradually degenerates and is finally completely absorbed by the 

 developing star (Compare figs. 22 and 31 lo.) 



The external form of the larva has been changed by the appearance of a number of elevations 

 and depressions iu its outer surface, the ciliated bands being elevated upon circular ridges (fig. 

 22, cb 1, 2, 3, and 4), while at points ou the sides of the disk beyond the end tentacles projections 

 in the ectoderm have made their appearance, these being the rudiments of the ophiurau arms (fig. 

 22, I, II, III, IV, and Y). 



The larval organ is cylindrical, but the disk has continued its dorsoventral flattening. 

 (Compare figs. 22 and 20.) 



The first and second ciliated bands are situated in the same places as in " E." The third, 

 while retaining its old liosition, has grown in upon the ventral disk toward the mouth (fig. 22, cb 3). 

 On the ventral side of the larva the fourth band has shifted from its old position behind tlie third 

 radial canal to one on the iiiterradii between arms II and III and III and IV. It has also become 

 iuterrupted on the oral disk iu a mamier similar to the third ciliated band (fig. 22, cb 4). 



The depressions before refeired to are caused by the invagination of the nervous system, which 

 structure has been forming since "D " in the thickened oral disk of ectoderm. Immediately below 

 the water ring and radial water canals the thickening has increased more rapidly than at other 

 points, thus producing a ring-shaped internal ridge, from which extend five radial thickened 

 ridges. These rudiments of the nerve ring and radial nerves bulge inwardly, no evidence of their 

 presence being apparent on the outside. When the thickening process has been completed the 

 whole nervous system gradually sinks in, leaving a circular groove from which five radial grooves 

 pass out. This is the stage in the formation of the nervous system which has been reached in 

 "F" (fig. 22, eg and rg). The invagination process begins at the ends of the radial nerves, just 

 inside of the curved tips of the end tentacles, the nerve ring being invaginated last of all. 

 (Compare figs. 23-27.) As development goes ou the edges of the grooves gradually close over the 

 nerves, the closure taking place in the same order as the invagination proceeded — that is, first 

 over the ends of the radial nerves, then finally, after gradually traveling up the radial nerves, 

 closing over the nerve ring. 



By the meeting and subsecjuent fusion of the edges of the grooves, part of their cavity 

 becomes cut oft' from the exterior and is left below the nervous sj'stem as the subneural space. 

 But this will be referred to again iu an older larva, in which the ])rocess of its formation is more 

 nearly completed, it having begun in a few only of the most advanced larva> of Stage " F." 



The nervous system shows a dilierentiation into two distinct layers, a fibrous one nearest the 

 water system and a cellular layer lying below the fibers (figs. 23-29). The nuclei of the cellular 

 layer are oval, with their long diameter perpendicular to the fibrous layer. 



Above the nervous system, separating it from the water system, is to be found the outer 

 periha-mal space (tigs. 23-30, opr). Recalling the condition of the perilncmal system in Stage "E,'' 

 we see that the ends of the interradial projections from the hypogastric and anterior enterocceles 

 have grown out over the nervous sj-stem, spreading in both directions until the outgrowths of each 

 interradial pouch meet those of its adjacent fellows in the radii over the origins of the radial 

 nerves; here the ends of the pouches fuse, and together they grow out over the radial nerves as 

 the radial perihaimal sinuses. 



In the starfishes, where the formation of this i)eriha>mal system has been observed, it is said 

 that no fusion takes place between the diverticula of the interradial i)Ouches of the hypogastric 

 and anterior euterocu^les when they meet in the radii, but that a mesentery is formed at the points 

 where the diverticula come in contact. This mesentery is described as continuing to the end of 

 the arms, separating the radial spaces into two parallel cavities. 



Nowhere could I find such a mesentery in sections of the larvae of 0. hrevispina, nor could 1 

 feel sure that it exists in the adult ophiuran. 



