1x6 



It seems to me that if, as de Selys Longchamps shews, the two cavities are continuous 

 in the eadier stages it is difficult to assume that they differ from one another to quite so great 

 an extent as is maintained by that author. Both cavities may in fact be said to be formed 

 from the original blastocoel, the mesoderm cells in one case arranging themselves round the 

 cavitv so as to give rise to a coelomic epithehum, and in the other case not behaving in this 

 manner. It must be remembered that at the metamorphosis of Actinotrocha, a considerable part 

 of the anterior region of the body, including the praeoral lobe, is completely lost. The balance 

 of evidence appears to be in favour of the view that the adult epistome is not a remnant of 

 the praeoral lobe (cf. de Selys Longchamps, 03, p. 42 and 04, p. 73). If then the anterior region 

 of the larva is not destined to give rise to an adult protomere (or proboscis), in consequence 

 of the peculiar phenomena which occur at the metamorphosis, it is perhaps possible that it may 

 have lost some of the characteristics of the protomere. The metasomatic cavity is admittedly 

 indistinguishable from the blastocoel in the younger larvae. If this be conceded, I see no great 

 difficulty in supposing that the cavity of the protomere of Actinotrocha remains in the same 

 undifferentiated condition. The praeseptal cavitv might thus represent not only the vascular 

 space which it is shewn to be by actual evidence, but also the region in which the anterior 

 body-cavity is potentially present. The praeoral lobe of Actinotrocha may thus possibly represent 

 the protomere of the Hemichordata. The region in Actinotrocha between the praeoral lobe and 

 the larval tentacles may represent the collar, as suppo.sed by Masterman, although the definitive 

 formation of its coelom is deferred to a late period in ontogeny. De Selys Longchamps (04) 

 throughout speaks of this as the "region collaire"; although, as he does not accept the homology 

 implied by that term, it may be presumed that he uses it for descriptive purposes merely. 



It is thus possible to imagine that in Actinotrocha, the anterior body-cavity never acquires 

 its tuil development, while the collar-cavity develops late; and that the adult Phoronis has lost 

 its proboscis, while its coUar-region is represented by the praeseptal part of the body, including 

 the lophophore and tentacles. 



It is perhaps a fact of some significance that the cavities of Actinotrocha have a 

 characteristic obliquity, the "trunco-collar septum", as de Selys Lonchamps calls it, extending 

 much further forwards dorsally than ventrally. The same obliquity is visible in the lophophoral 

 cavity which is developed late in larval life, since this rests on the anterior face of the main 

 septum, a relation which is well shewn by Goodrich (03, fig. 2). A similar obliquity of the 

 coelomic cavities to the actual main axis of the animal is obvious in Cep/ialodisciis, and is 

 shewn for instance in the sagittal section of a bud figured in PI. XIII, fig. 174 of this Report. 



If I am right in the explanation I have given in Sect. VIII (p. 30) of the morphology of the 

 arms and operculum of Cephalodiscus (cf. PI. III, fig. 25; PI. XII, figs. 158 — 160), the entire 

 anterior border of the collar of that animal is prolonged into a free fold, with the exception 

 of two lateral regions where there may be an interruption in the fold, and of the mid-dorsal 

 region. The ventral half of this fold is constituted by the operculum, and the dorsal half 

 by the tentaculiferous arms. If the lophophore of Phoronis belongs to a region which is 

 morphologically comparable with the collar of the Hemichordata, it may be suggcsted that its 

 tentacles should be compared not merely with the arms of Cephalodiscus, but with the whole 



