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 Avas 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 Adinotroclia, between 

 the mouth and the anus 

 (Fig. 4 C, w, 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 Avhich no homo- 

 logue can be found in the 

 AdmotrocJia or the Troclio- 

 pliore stage of other groups. 



We may thus, perhaps, 

 recognise in the Ectoj)roctous 

 larvae a somewhat modified 

 TrochojjJwre stage, which in 



the possession of a ventral 



sucker may be connected with the Adinotroclia 



Fio. 2S. — Diagrammatic median section tlirough a 

 Phylactolaomatous Bryozoan (constructed after 

 CoRi). This should be compared with the dia- 

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

 (Fig. 41, p. 80). o, anus; ed, hind-gut; eli, epis- 

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

 ganglion; Ih, lophophore- cavity, here termed 

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

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

 tentacular membrane ; ts, tentacle-sheath. 



Eecently, however. 



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

 Molluscau larvae, with which it lias repeatedly been liomologised. 



