The Development of Ischnochiton. 637 



phore was a quadriradially symmetrical organism 

 whose principal axis corresponds to that of the 

 gastrula, and that the shifting of this axis is second- 

 ary i). 



The original ancestor therefore may be compared to a highly 

 modified gastrula, or gastraea so far at least as these three groups 

 of animals are concerned, in which the mouth was situated at the 

 centre of the lower hemisphere, and an ectodermic pharyngeal portion 

 of the alimentary canal (stomodaeum) existed in connection with the 

 «ntodermic mesenteron. At the opposite end an apical sense organ 

 probably existed which was situated at the intersection of the quadri- 

 radial nervous system occupying the upper hemisphere. Encircling 

 the entire animal was an equatorial band of cilia. All of these features 

 appear during development in varying degrees of perfection, and 

 possibly others may have existed originally but embryology gives 

 no certain answer regarding their character. Subsequently in the 

 phylogeny the mouth shifted its position , due it is believed to the 

 advantage of coming into close proximity of the prototroch which 

 served as a food procuring apparatus '^). 



Many developing embryos during the first stages of this shifting 

 show a narrow slit extending posteriorly from the blastopore which 

 is looked upon by some investigators as indicating an ancestral 

 character but this is by no means apparent. In several cases this 

 feature scarcely exists at all and where it is strongly developed it 

 appears to be secondary ^). And neither does it appear correlated 

 with the formation of the anus, at least not in Chiton, where this 

 latter organ does not form until fully fifteen days have elapsed. 

 Ultimately the mouth became situated immediately posterior to the 

 prototroch, and thereby the former radial symmetry became in large 

 part modified in later stages, although traces of it still existed, as 

 development of modern forms clearly shows. 



Returning to the phenomena of early development it may be 

 said that since they undergo a similar shifting of axes in their deve- 

 lopment, it is reasonable to suppose that Polyclades, Annelids and 



1) Cf. Hatschek ('88), Lang ('81), Selenka ('81) and others. 



2) Cf. KoKscHELT und Heidek, Entwicklung der wirbellosen Thiere, 

 p. 1162. 



3) Child ('97) has recently shown that the lips of the blastopore 

 concresce owing to the growth of the somatic plate. 



