No. 3.] GROWTH AND METAMORPHOSIS OF TORNARIA. 443 



similar process to that found in Auricularia. The histology of 

 the ciliated band is exactly the same in the two larvae, and the 

 continuity between the cilia of the longitudinal band in the 

 region in front of the mouth with the ciliated upper dorsal sur- 

 face of the oesophagus may in some degree represent a similar 

 fusion found in Auricularia and recently described by Semon. 

 Lastly, the sudden and very great diminution in size of the 

 larvas at the time of metamorphosis is unique ; and as we have 

 an exactly similar phenomenon in the metamorphosis of the 

 Holothurian larva into the young Holothurian (Pentactula), we 

 may, I think, add this to the other resemblances. 



The differences between the larvae have been pointed out in 

 the preceding section. The apical plate and eyes seem to be 

 absent in the Auricularia, although there may be a rudiment (?) 

 of the first of these in the Echinoderm larva. The muscle 

 band from the apical plate in Tornaria has no homologue in 

 Auricularia. The presence of two posterior pairs of enterocoels 

 in Tornaria is not readily to be explained, and is, it seems to me, 

 the one great difficulty in our comparison. The anterior entero- 

 coel in the Holothurian, for instance, gives rise not only to the 

 same structure as the anterior enteroccel of Tornaria, but also to 

 the posterior body cavities of the Holothurian as well. The 

 problem of enteroccel formation and its apparently close connec- 

 tion with metamerism is still one of the unsolved problems of 

 morphology, and until we have a clearer knowledge of the 

 meaning of the phenomenon, all detailed comparisons in which 

 the subject is involved must remain open. The large posterior 

 circular band of Tornaria seems to have no homologue in Auricu- 

 laria, and is apparently a structure sui generis. 



But what seems to me to be the most essential point in the 

 comparison of Tornaria and Auricularia is one that is not gener- 

 ally emphasized ; viz. that it is the young Tornaria that resembles 

 most closely the Echinoderm larva, and not the older Tornaria. 

 If we compare the young Tornaria before the development of 

 the circular band with the other larva, the resemblance amounts 

 almost to an identity, so that the larvae are almost indistinguish- 

 able. This identity cannot be explained by an adaptation to sim- 

 ilar conditions of life in the two forms, for the reason that the 

 most essential and vital points of comparison relate to the 

 internal organs. The anterior body cavity with its left dorsal 



