94 ENTOI'ROCTA. 



the cup-like aboral wall of the larva, and consists of a rather deep 

 ectodermal depression, part of the anterior section of which can be 

 evaginated. 



In the comparison which has often been made between the Pedicellina larva 

 and the larvae of the Ectoprocta (especially with Cyphonautes), these two pro- 

 visional organs have played a conspicuous part. The ciliated disc has usually 

 been regarded as the homologue of the retractile disc of the Ectoprocta. Like 

 the latter, it is retractile, and seems to function as a sensory organ. This, at 

 least, seems to be proved by the observation that the larva of Pedicellina, whet 

 swimming, always carries this organ directed forward. The structure of the 



Fig. 45.— Two later stages in the development of Pedicellina (after Hatschek, from Balkour's 

 Text-book), a, anus ; an.i, anal invagination ; /</, ciliated disc ; hg, hind-gut ; I, liver ; m, 

 mouth ; nph, nephridium ; v, vestibule ; x, dorsal organ. 



dorsal organ might lead us to conclude that it was homologous with the pear- 

 shaped organ of the Ectoprocta, but the difference in the position of the two 

 organs relative to the ciliated ring must be taken into consideration, and 

 such a comparison must be made with caution. The organ under consideration 

 lies in the one case in the aboral, and in the other in the oral region. In con- 

 sequence of the difficulties which, according to the most recent researches, stand 

 in the way of comparing Pedicellina and the Ectoprocta (p. 101), it must be 

 regarded as doubtful whether we are in any way justified in searching for 

 homologies of this kind between the larvae of the two groups. 



The dorsal organ (Figs. 44 and 45, x) has, up to the present, been interpreted 

 in very various ways. HatscHBK, who thought that he had convinced himself 

 that an entodennal sac derived from the rudiment of the mid-gut passed into 



