30 HEMICHORDATA CHAP. 
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 Actinotrocha 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 eversion 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-shts 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 gill-slits 
connected with the pharynx; but the view was first defended in detail by 
Bateson. 
* See, for example, Minot, Amer. Nat. xxxi. 1897, p. 927. 
