SYNAPTA VIVIPARA. 79 



(Fig. 52) and two large and three small smooth holes. On the side of the plate next 

 to the anchor and near the posterior end is an arched bow, which bears a few teeth on 

 its anterior edge. Increased growth of the plate often increases the number of holes 

 (Fig. 53), but as a rule they are very constant. The calcareous rods which were so 

 abundant in the tentacles of the young larvae are so few that for a long time I was led 

 to consider them entirely wanting. The tentacles (Fig. 57) of the adults are long and 

 slender with from 12 to 18 pairs of digits, but the number varies greatly with the age 

 and size of the animal. The glandular organs which characterize the young ten- 

 tentacled stage seem to be entirely wanting now ; at any rate, I have never found any 

 trace of them in an adult. 



A number of interesting monstrosities were found, chiefly among the older embryos. 

 One of these is shown in Fig. 93, but some of the others were much more complicated, 

 consisting of three, four, and, in one case, five young, which had grown together, or 

 budded from each other in various ways. Among adults, besides the rather common 

 addition of an extra tentacle, the only peculiar specimen found was one which -had only 

 three radial muscles and nerves and only eleven tentacles. There were three tentacles 

 in the mid-dorsal interradius (indicated by the mesentery), and four tentacles in each of 

 the lateral interradii. 



9. CONCLUSIONS. 



Probably no theory of echinoderm phylogeny has attracted more attention or seems 

 more plausible than that upon which Semon ('88) determined, as the result of his 

 studies on the development of the auricularia larva, of Synapta digitata. Although it 

 still finds supporters at the present time, the investigations of Ludwig ('91) on Cucu- 

 maria and of Ludwig and Barthels ('91) on the anatomy of the Synaptidae have shown 

 the incorrectness of Semon's views, while the observations of Bury ('89 and '95) have cast 

 doubt on his interpretation of some of the conditions in auricularia. It is not my inten- 

 tion to enter here into any discussion of the phylogeny of the echinoderms but only to 

 suggest some of the points in the phylogeny of the holothurians, upon which the history 

 of Synapta vivipara seems to throw some light, and to indicate some of the particulars in 

 which my studies have apparently offered support to Bury's ('95) theory of the ances- 

 tral form of the echinoderms. 



There are three possible opinions concerning the relationship of the Synaptidae to 

 the other holothurians : first, Semon's ('88) view that Synapta represents a primitive 

 form, from which the other holothurians have been derived; second, Cuenot's ('91) view 



