66 HUBERT LYMAN CLARK ON 



which we have seen was formed from the floor of the atrium. This ring is the 

 beginning of the central nervous system, and from it the tentacle and radial nerves arise. 

 As the primary tentacles push upward past the atrium, they lie closely appressed to its 

 floor and wall. They retain this position even after the circumoral ring is pretty clearly 

 defined, and before they have grown much above it, the radial nerves appear between 

 each pair of them as outgrowths of the central ring. These outgrowths pass directly 

 over the secondary outgrowths of the water-vascular system and bend backwards to run 

 toward the aboral pole of the body. Very soon after they appear (not before them, as 

 Semon ('88) describes for S. diyitata), that part of the circumoral ring appressed to each 

 primary tentacle begins to grow upward with it on its inner side, forming the tentacle 

 nerves. As the tentacles continue to grow and press the anterior body wall outward, the 

 ectoderm which covers them at the tip becomes noticeably thickened, especially on the 

 outer side (Fig. 54), and apparently assumes a sensory function, probably in connection 

 with the tentacle nerve. The formation of the otocysts of Thomson ('02), the " horor- 

 gane " of Baur ('64), takes place as described by Semon ('88). They arise by evagina- 

 tions from the outer side of the circumoral ring close beside the outgrowths which form 

 the radial nerves. With the growth of the latter, the otocysts come to lie external to 

 them at the point where they bend backward, and in this position the sense-organs remain 

 throughout life. It is a very evident and noteworthy fact that the development of the 

 radial nerves and otocysts does not take place at the same time in the five radii, but there 

 is a marked difference between them. The first to appear is that nerve which subse- 

 quently indicates the mid-ventral radius, and with it appear its two otocysts. The two 

 lateral ventral nerves appear next, and witli them the otocyst which accompanies each one 

 on its ventral side. The lateral dorsal nerves next appear (Fig. 1), and very soon after- 

 wards the two other otocysts of the right and left ventral radii are formed. Last of all 

 to develop are the otocysts of the right and left dorsal radii. This sequence in the 

 appearance of these nerves and sense-organs is probably connected with the fact already 

 mentioned, that the thickened ectoderm which made up the floor of the atrium did not lie 

 symmetrically around the mouth, but the greater part of it was posterior or, when the 

 mouth lies at the anterior end of the larva, ventral to it. What the significance of this 

 condition may be, I am unable to suggest, but it is interesting to note that in Cucuniaria 

 Ludwig ('91) found the ventral radius the most advanced in development. Any possible 

 similarity ends here however, for the development of the other radii was quite the reverse 

 in Cucumaria of what it is in Synapta. 



About the time of the completion of the pentactula form, there appear in the ecto- 

 derm of various parts of the body peculiar invaginations (Fig. 41) -which are finally 



