30 HEMICHORDATA 



the tips of the crescent nearly meeting dorsally, so as to con- 

 stitute an almost complete ring following the bases of the 

 tentacles, into each of which it gives off a blind outgrowth. At 

 the metamorphosis, the crescentic space becomes the prae-septal 

 body-cavity and the cavities of the tentacles of the adult, the 

 circular blood-vessel of which is formed from the remains of the 

 large prae-septal space of the larva. Schultz, in calling atten- 

 tion to the fact that both Phoronis and its larva have a striking 

 power of regenerating lost parts, confirms the conclusion that 

 this animal belongs to the Hemichordata. He gives reasons, 

 however, for believing that it is in the adult Phoronis rather 

 than in the larval Actinotrooha that it is possible to discover 

 the most satisfactory evidence of this affinity. 



The metamorphosis ^ of Actinotrocha is very remarkable, and 

 is accompanied by the e version of a ventral ingrowth of the 

 body -wall. A loop of the alimentary canal passes into this 

 eversion, which becomes the main part of the body of the adult ; 

 and the anus is thereby brought relatively nearer the mouth 

 than in the larva. The occurrence of this process may help to 

 explain the position of the anus in the Pterobranchia. 



Affinities of the Hemichordata. — There can be no doubt 

 that some of the resemblances, in structure and in development, 

 between Balanoglossus and certain Vertebrates are extremely 

 striking. The view that Balanoglossus is related to the ancestors 

 of Vertebrates ^ appears to exclude other views ' which have been 

 suggested with regard to the same question. The Balanoglossus- 

 theory does not explain the similarity between the segmentation 

 and the excretory systems of Vertebrates and Chaetopods ; but, 

 on the contrary, there are important characters which Vertebrates 

 share with Balanoglossus but with no other " Invertebrates." 

 Of these the most important appear to be the resemblances 

 between the gill-slits and gill-bars of Balanoglossus and 

 Amphioxus ; the position, structure and mode of development of 

 the central nervous system ; and the presence of a structure in 

 the Hemichordata, which may be regarded as the notochord. 



1 Vol. II. p. 459. 



^ Huxley, in 1877 (Man. Anat. Invert. Animals, p. 674), proposed to unite the 

 Enteropneusta with the Tunicata as Pharyngopneusta, in allusion to the o-ill-slits 

 connected with the pharynx ; but the view was first defended in detail by 

 Bateson. 



^ See, for example, llinot, Amer. Xat. xxxi. 1897, p. 927. 



