72 Evolution and Distribution of Fishes 



Our knowledge of the embryology of freshwater and 

 land nemerteans is still too imperfect to permit definite 

 statements as to the formation of mesenteric folds for hang- 

 ing of the alimentary canal in place, after the manner seen 

 in vertebrates. But the formation of paired "imaginal 

 discs.," and the ingrowth in bilateral fashion of the posterior 

 pair of these in the embryo, recalls the closely similar mode 

 of formation of the mesoblastic mesenteries in vertebrates. 

 Burger's statement however, as quoted below (p. 93), 

 seems to indicate an exact homology. 



VIII. The proboscis and proboscis sheath. 



From their mode of origin, and their close relation to 

 the anterior part of the alimentary canal, the proboscis and 

 the proboscis-sheath or rhynchocoel can next be studied. 

 This double structure is undoubtedly anticipated and led 

 up to in the corresponding though simpler structure of the 

 rhabdocoel turbellarians. The great importance of the 

 sheath phylogentically consists in its being evidently the 

 evolving predecessor of the notochord, while the proboscis 

 evidently becomes modified into the pituitary body, as well 

 as various buccal structures; a very brilliant generalization 

 this, for which we are mainly indebted to Hubrecht. 



Zoologists are agreed that the proboscis represents an 

 ectodermal invagination or introvert near the anterior head 

 region. But two different modes of origin might be claimed 

 for it. Either it is an in-pitting from the buccal region 

 within and behind the mouth — and this conforms to its 

 relation in turbellarians, as well as in freshwater, in land, 

 and in not a few marine nemerteans — ; or it is an invagina- 

 tion of the anterior cephalic region outside of and above 

 the mouth, as in many marine nemerteans. The writer has 

 advocated the former as the true interpretation, for all 

 transition stages from an intraoral to an extraoral position 

 can be traced in living marine types, which for many reasons 

 the writer regards as derivative from freshwater forms. 

 Figure 2 then would represent the primitive condition. 



But with progressive elaboration, elongation, and 

 sensitivity to environal agents, the inverted structure grad- 

 ually evolved an inner highly glandular half that could 



