GENERAL CONSIDERATIONS. 



57 



supported by the utilisation of this structure as larval tactile organ, 

 obtains further confirmation from the statements of Harmer and 

 Prouho quoted above (p. 20). Both these authors were able to 

 observe that the larval brain is connected with the retractile disc, 

 and Prouho was able to add the information that the latter organ 

 underwent, during metamorphosis, a degeneration similar to that 

 which takes place in all other , 



larval organs (p. 24). 



Of the larval organs lying 

 on the oral side, the so-called 

 sucker is evidently the homo- 

 logue of the invagination 

 which lies on the ventral side 

 of the Adinotrocha, between 

 the mouth and the anus 

 (Fig. 4 C, iv, p. 7)* In the 

 Bryozoa also, during meta- 

 morphosis, this invagination 

 gives rise to a part of the 

 body-wall, though it here 

 produces only the adhesive 

 plate which lies altogether 

 at the base of the primary 

 zooecium. The structures 

 known as the ectodermal 

 furrow and the pyriform 

 organ, on the contrary, seem 

 to be provisional organs 

 peculiar to the Bryozoan 

 larvae, for which no homo- 

 logue can be found in the 



Adinotrocha or the Trocho- 

 phare stage of other groups. 

 We may thus, perhaps, 

 recognise in the Ectoproctous 

 larvae a somewhat modified 

 Trochophore stage, which in 

 the possession of a ventral 

 sucker may be connected with the Adinotrocha 



Fio. 28. — Diagrammatic median section through a 

 Phylactolaematous Bryozoan (constructed after 

 Cora). This should be compared with the dia- 

 grams of Phoronis (Fig. 5, p. 9) and Terebratula 

 (Fig. 41, p. 80). a, anus ; ed, hind-gut ; eh, epis- 

 tomal cavity ; ep, epistome ; /, funiculus ; g, 

 ganglion; Ih, lophophore- cavity, here termed 

 the circular canal ; m, mouth ; ma, stomach ; 

 n, nephridium ; oe, oesophagus ; i, tentacle ; tm, 

 tentacular membrane ; ts, tentacle-sheath. 



Recently, however. 



* In position this organ agrees closely with the rudiment of the foot in 

 Molluscan larvae, with which it has repeatedly been homologised. 



