46 



St. Andrews I have examined a large number of specimens during 

 and after metamorphosis, and in most cases aided hj sections 

 I came to the conchision that the hood was converted into the 

 posterior moiety of the oral field of the adult. It is a matter 

 of Httle moment morphologically, for the whole tentacular region 

 may be thrown off and renewed during adult life. 



During the metamorphosis the changes in the disposition of 

 the mesentery and the splanchnopleure lead to the dorsal vessel 

 of the larva already in communication with the blastocoele being 

 comiected with the lateral vessel, and the latter is also extended 

 to the blastocoele, bifurcating below the septum. The dorsal 

 vessel becomes the afferent vessel, and the lateral vessel the 

 efferent vessel of the adult. 



In the newly metamorphosed Phoronis, the nephridia have 

 lost their solenocjrtes, but what becomes of them I cannot say. 

 The nephridia he between the sei^tum and the wall of the lopho- 

 phoral cavit^^ At this stage they have not yet penetrated the 

 metasomal cavitj^ 



5.— POSITION OF THE PHORONIDEA. 



Phoronis is a genus, the developmental and structural features 

 of which render it worthy of class distinction at least, and in 

 consequence many attempts have been made to indicate to which 

 group it ought to be relegated. So far practically every worm 

 group has been suggested and Polj^zoa and Brachopoda, and 

 recently particularly the attempt has been made to prove that 

 Phoronis is allied to Balanoglossus and other so-called Hemi- 

 chordates. Masterman, attracted by the general resemblance 

 of the larva to that of Balanoglossus, has tried to show that 

 Actinotrocha has notochordal structure in the digestive glands, 

 and has proceeded to find coelomic cavities and other features 

 homologous with those of the Enteropneusta and their alHes. 



The trouble may be said to have begun when Bateson identified 

 the anterior diverticulum of the alimentary canal of Balanoglossus 

 with the vertebrate notochord. This structure which admittedly 

 has a supporting function is quite obscure as to the laj^er from 

 which it arises, and it certainly is not developed as the vertebrate 

 notochord is developed. In the Urochordata, in the Cephalo- 



