322 AETHUR WILLEY. 



The time or order of formation of certain organs seems to be 

 very generally subject to a great deal of variation. I have 

 previously described some such variations in the case of the 

 secondary gill-slits of Amphioxus, and similar instances are 

 very easy to find. I therefore suggest that either a slight 

 delay in the formation of the mouthy or an acceleration in the 

 anterior development of the notochord — probably the latter, — 

 introduced in the first place as a variation and subsequently 

 becoming a fixture, was the method by which the perforation 

 of the mouth at a point other than the primitively dorsal one 

 was rendered possible. 



The above observations and considerations all tend to show 

 that the primitive vertebrate mouth, before the cranial flexure 

 had become an established feature of the vertebrate ontogeny, 

 had a dorsal or a dorso-terminal position. 



In view of this conclusion a genuine difficulty is presented 

 by the position of the mouth in Balanoglossus, where it is from 

 the beginning ventral. This difficulty cannot be fully met at 

 present, but it may be well to point out that the intermediate 

 stage between a ventral mouth as found in Balanoglossus, and 

 a dorsal one as it occurs in the Ascidian tadpole, would be 

 arrived at by a form in which, by a reduction of the prseoral 

 lobe, the mouth came to occupy a terminal position. Supposing 

 it possible to conceive a common ancestor for all the Proto- 

 chordata, it would seem to be probable that it had a terminal 

 mouth. For from such a situation the mouth could be made 

 to assume either a definitely dorsal or ventral position, accord- 

 ing to circumstances, as soon as the paired head-cavities 

 co-operated to produce the peculiar features and proportions of 

 a prseoral lobe. 



Appendicularia is a form in which, together with the 

 reduction, and indeed apparent absence in the adult of any 

 trace of a prseoral lobe — a state of things brought about by the 

 purely pelagic life which has been acquired by the organism — 

 the mouth has come to occupy a terminal position, and thus 

 shows us that, under certain circumstances, the topographical 

 relations of the mouth which I have just predicated for the 



