GENERAL CONSIDERATIONS. 



101 



/ 



ms 



of future investigators. Beside the points of agreement with the Trochophore, 

 the Entoproctous larva shows many points of comparison with the Ectoprocta, 

 which may also be traced back to the Trochophore type. We have, however, 

 already shown (p. 94) that such a comparison meets with considerable difficulties 

 in the details of the organisation of the two larvae. 



It is of great importance for a comprehension of the adult Pedi- 

 cellina (Fig. 48) that we should carefully consider the metamorphosis 

 of the larva, made known by Barrois and Harmer. Such an 

 examination shows that the relative positions of the mouth, the 

 anal aperture, the alimentary canal, the ganglion, and the nephridial 

 canals undergo no alterations during metamorphosis. This latter 

 consists solely in the rotation 

 of the whole complex of organs 

 now under consideration round 

 a transverse axis. The vesti- 

 bule alone undergoes any con- 

 siderable change, part of it 

 being dilated and part degen- 

 erating, a new aperture also 

 forming. It thus results, from 

 a careful consideration of the 

 metamorphosis of Pedicellina, 

 that the adult is here at essen- 

 tially the stage of organisation 

 of the Trochophore. We are 

 thus compelled to regard the 

 short line extending from the 

 mouth to the anal aperture in 

 the adult Entoproctan as the 

 ventral median line, for this 

 still directly corresponds to the 

 region at which the slit-like 

 blastopore closed. We must, 



then, for the sake of consistency, regard the ganglion lying at this 

 point (n) as a sub-oesophageal ganglion belonging to the ventral side. 

 In the same way, we would then, perhaps, be able to trace back 

 the ring of tentacles (t) of the adult to the pre-oral ciliated ring 

 of the larva. 



The above considerations seem to show that the Entoprocta, in spite of many 

 remarkable points of agreement, cannot in any way be compared with or 

 combined into the same group as the Bryozoa (Ectoprocta). We regard them as 

 an entirely independent group, and in this respect agree with Hatschek 



Fig. 48.— Diagrammatic median section through 

 an adult Pedicellina (after Ehlers). a, anus ; 

 ep, epistome-like oral fold ; ex, excretory organ ; 

 g, genital organ ; m, mouth ; ms, fibres of the 

 sphincter muscle ; n, nervous system ; t, ten- 

 tacles. 



