42 THE EVOLUTION OF THE META20A 



I am fully convinced that this, my interpretation, is right in 

 spite of a recent discovery of Xeopilina which has been found 

 bv Lemche (1958) to be a segmented animal with five body 

 segments. As in the case of several other Ameria we can 

 observe a partial ^o/yw^r/^^Z/c/; in a certain number of Mollusca, 

 and therefore not only in Neopiliria and in the related fossil 

 species. It occurs, however, in yieopilina to a stronger and fuller 

 degree than, for example in Tetrabranchia among the Cepha- 

 lopoda. 



As far as is known, no pelagic larvae have been developed 

 by the acoelous Turbellaria in spite of the fact that they live 

 (^^-ith a few exceptions in planktonic species) on the sea bot- 

 tom. Unfortunately, our present knowledge of the ontogeny of 

 Acoela is still very unsatisfactory. Surprisingly enough, we do 

 not find pelagic larvae in Turbellaria earlier than in Polycladida 

 (e.g. Miiller's larva). It is aproctous like the grown-up Tur- 

 bellaria— it has no anal orifice— and for this reason it cannot 

 be compared to a trochophore but rather to a protrochophore, 

 unless as it has even been thought, we consider the aproctous 

 condition to be secondary. This larva has developed its own 

 swimming apparatus in the form of an irregular circle which 

 consists of eight lobular excrescences that are rimmed with 

 ciHa. This larva is poised "vertically" in water. It develops 

 gradually and without a real metamorphosis into a polyclad 

 that Hves on the sea bottom. It is not my intention to discuss 

 here any other larval groups that live in plankton; the only 

 form I would like to mention is the so-called pilidium of 

 Nemertinea. In spite of the fact that Nemertinea stand close to 

 Turbellaria, they have nevertheless become euproctous ; there 

 is a very great difference between the pelagic larvae of the two 

 groups which can be observed not only in their external forms. 

 The Pilidium is aproctous— in agreement with its ontoge- 

 netic stage— and it has a completely different type of deve- 

 lopment to the Miiller's larva. On the basis of these facts we 

 can come to the conclusion that the two larvae developed 

 independently from each other, and, furthermore, that they 



