452 EMBRYOLOGY 



parts, so that fifteen tentacles can now be recognized. 

 By the addition of two new tentacle-buds to each of the 

 five groups, the number of tentacles soon increases to twenty- 

 five, arranged in five radial groups. The tentacles (Figs. 219 

 and 220 t) project into the vestibule, the roof of which is 

 stretched out between the upper margins of the oralia. 

 This roof is at first thick, but gradually becoming thinner 

 (Fig. 220 r), finally disappears entirely. The gradual dis- 

 appearance of the roof is partly a result of growth, partly 

 brought about by histolytic processes. According to Bury, 

 such processes can also be recognized on the rest of the 

 larval body, and cause a disappearance of the histological 

 differentiation. Probably migratory cells make their appear- 

 ance in this connection as ^phagocytes. 



After the disappearance of the roof of the vestibule, the 

 tentacles, on which papillae subsequently bud forth, project 

 free to the exterior (Fig. 221 C). The under-part of the 

 larva has elongated into the stalk, and it now rests with its 

 terminal plate on some support. The fundaments of the 

 arms bud forth on the upper part of the cup as five pro- 

 jections (Fig. 221 C). The tips soon split into two branches 

 corresponding to the permanent forking of the arms. One 

 of the radial tentacles, each of which has likewise split into 

 two, unites with the fundament of each of the arms. Sur- 

 rounded by this, it grows out with it and becomes the ambu- 

 lacral canal of the arm. By means of lateral budding it 

 gives rise to the tentacles of the arm. The tentacle which 

 is first formed always remains at the tip of the arm. The 

 new tentacles arise at its base in groups of three. The mode 

 of formation of the tentacles is therefore similar to that of 

 the ambulacral feet of other Echinoderms. The development 

 of the pinnules is the result of a forking of the arms, which 

 occurs alternately to the right and to the left (W. Carpenter, 

 Perrier). This explains the alternating position of the 

 pinnules. 



Important changes have taken place in the skeleton of the 

 larva. Between the basal and oral plates, alternating with 

 the latter, five new skeletal pieces, the radialia, have made 

 their appearance (Fig. 223^ ) ; these become greatly enlarged, 



