RAYS AND BILATERAL SYMMETRY 255 



ORIGIN AND GROWTH OF NEW RAYS. 



Two or three ambulacra are the first external evidence that 

 one sees of the incoming ray. These appear in a cleft, or more 

 properly, a pocket, on the oral side of the disc, and extend out- 

 ward from the peristome (tig. 5). This is much nearer the oral 

 edge of the peristome than Perrier, '93, describes the base of 

 the newly forming ray to be in Labidiaster. A little forcing 

 with a probe at the distal end of the pocket discloses other 

 ambulacra, small, but distinctly formed. The pocket soon 

 lengthens and broadens, revealing more ambulacra, but as yet 

 nothing can be seen of the new ray when the animal is looked 

 at on its aboral side. Until a considerable number more of am- 

 bulacra are formed, and the ray projects to some extent from 

 the edge of the disc, it is bent down at its tip over the newly 

 forming tube feet. At the beginning the feet appear to be in a 

 single row, but sections prove them to be in pairs from the very 

 outset. 



At first no spines are present, but they soon appear, the 

 furrow-spines first, in form and arrangement corresponding with 

 those of the old rays. As the ray develops the interradial dorsal 

 wall of the disc is carried out, so to speak, to form its dorsum. 



From this outline of the early development, it will be gathered 

 that at the outset and for some time the development pertains 

 wholly to structures belonging to the ventral portion of the ray : 

 the floor of the ray, the ambulacral tentacles, the radial water 

 tube, the radial perihasmal tubes, and the radial nerve band. 

 For some time the only part of the ray that exists may be said 

 to be embedded in the interradial tissue. Fig. 9 is a diagram- 

 matic representation of a vertical interradial section showing a 

 new ray at a stage somewhat earlier than the one shown in fig. 

 5 ; and fig. 5 represents a ray considerably younger than Ix^ and 

 rx^ in figs. 3 and 4. 



Between what we suppose to be the initial step in the pro- 

 duction of a new ray, and a condition in which ambulacral 

 out-pocketings on the radial water-tube are clearly indicated, 

 we have not succeeded in finding stages. However, from the 

 stages which we have, a tolerably complete picture can be ob- 

 tained of the entire course of things. 



