72 HUBERT LYMAN CLARK ON 



growth" has grown up on one side or the other of the mid-ventral nerve, although in 

 normal specimens it does not develop at all. Occasionally the extra tentacle is in the mid- 

 dorsal interradius, and in such cases it is probable that the left dorsal " secondary 

 outgrowth," which normally develops only a single tentacle, has given rise to a second 

 on the dorsal side of the left dorsal nerve. Corresponding to these changes in the num- 

 ber of tentacles additional plates appear in the calcareous ring, but these plates do not 

 arise by interpolation of new rods. On the contrary, the plate of the same radius with 

 the new tentacle increases its length and sends upward a new projection for the support 

 of the tentacle (Fig. 47), and this subsequently forms the center of the new accessory 

 plate. In specimens of the calcareous rings cleaned with caustic soda, it seemed to me 

 that the calcareous plates of the right and left dorsal radii were less easily separated from 

 those plates on their ventral side than from those on the dorsal, so I am inclined to think 

 that for a time, if not throughout life, these two plates remain in closer union than any 

 of the others. In specimens with thirteen tentacles, there is an additional plate in the 

 calcareous ring corresponding to the extra tentacle. About this same time the miliary 

 granules (Fig. 50) begin to appear in various parts of the body-wall and in the tentacles. 

 Like all other calcareous concretions of Synapta, they are formed by mesenchyme cells. 

 They usually appear in clusters of several hundred, which continue to increase in number 

 afterwards until it may reach thousands. By the time twelve tentacles have appeared, 

 the genital gland begins to push over on the left hand side, but it is not until long after 

 the adult form is assumed in all other respects that the left branch of the gland equals the 

 right in size. As soon as the left branch is well started, the germinal epithelium begins 

 to push upward in the mesentery beside the stone-canal, and forms the genital duct, but 

 does not reach the outer body-wall for some time. This account of the development of 

 the genital duct accords with Mortensen's ('94) observations on Cucumaria glacialis, 

 although his account of the origin of the genital gland itself differs considerably from my 

 observations. Important changes are going on meanwhile in the nervous system. From 

 the inner side of the circumoral ring nerves or bands of nervous tissue arise and pass 

 inward to the oesophagus. These will be referred to more fully in describing the nervous 

 system of the adult. Even in the ten-tentacled stage, before the remaining two tentacles 

 have made a fair start, there arises on each side of the tentacle nerve at its base a knob- 

 like outgrowth which becomes covered over with a peculiar mesenchyme layer, and these 

 form the " eyes," which also appear at the base of the eleventh and twelfth tentacles, 

 after they receive their nerves from the circumoral ring. With the appearance of these 

 eyes, the first trace of pigment appears in the mesenchyme not only about them but in 

 various parts of the body, especially around the calcareous ring. This pigment on its 



