xiv PREFACE. 



Vorderdarm, jMitteldarm, Hinterdarm, Kopfdarm, has caused nie some 

 perplexity. It has been variously rendered in the translation by "gut," 

 "enteron," "enteric tube," "alimentary canal," "digestive tract." The 

 fact is that, whilst we have no definite nomenclature at present in use . 

 in English which recognises the true morphology of the canal which com- 

 mences with the mouth and ends with the anus, the nomenclature in use in 

 Germany is of very doubtful advantage, since it has not a sound morpho- 

 logical basis, but is altogether superficial. " Darm," for which our readiest 

 equivalent is " gut," is used indifferently for the whole or for any part of 

 the physiological entity which reaches from oral to anal aperture. Eut the 

 Eno-lish word " gut " is associated rather with the hmder than with the fore- 

 most portion of this tract. It Avill probably be foimd most convenient to 

 speak of the physiological whole as the "alimentary canal," or "digestive 

 tube ;" and those terms I have endeavoured consistently to make use of in 

 this sense, though sometimes the term "enteric tube" has been similarly 

 applied. 



The division of this tube or canal into pharynx, oesophagus, stomach, 

 and intestine ; or, again, into fore-gut, mid-gut, and hind-gut (Vorderdarm, 

 Mitteldarm, Hinterdarm, p. 48), is one based upon superficial adaptations 

 of form, and does not admit of a comparison of the parts so designated in 

 the various phyla of the Animal Kingdom. The pharynx and the oeso- 

 phagus of the Vertebrata are developed from the endoderm of the embryo ; 

 the parts which receive tlie same names in the MoUusca and the Arthropoda 

 are developed from the ectoderm. The hind-gut of the Vertebrate is endo- 

 dermal in origin, ectodermal in the Arthropod, and partly eudodermal 

 partly ectodermal in the MoUusca. In fact there is no attempt to recog- 

 nise the facts of embryology in the terminology applied to the alimentary 

 canal. 



Under these circumstances I have proposed (Quarterly Journ. Microsc, 

 Science, April, 1876, and " Xotes on Embryology and Classification," London, 

 1877, p. 11), to distinguish the primitive digestive space which develops 

 from the endoderm (in fact the gastrula-stomach) as the " enteron." 

 The anterior passage leading into this from the mouth, and formed hj 

 an ingrowth of ectoderm, I have termed the " stomodaeum," and the 

 corresponding passage leading from the anus I similarly propose to call 

 the "proctodffium." These three primary factors of the alimentary tract 

 are most equally developed in the Artlu'opoda and some MoUusca. In 

 Vertebrata the stomoda3um is exceedingly smaU, if indeed its true homo- 

 logue exists at all (excepting in the Timicata). The proctodtevun is also in 

 them evanescent. The middle portion of the alimentary tract formed from 

 the primitive enteron (archenteron), which does not entirely coincide with that 

 part to which the term " Mitteldarm " is applied, does not in all the various 

 animal phyla take up the whole of the primitive enteron. This, in fact, 

 only occurs in some of the Coelenterata, which may therefore be said to 

 possess in the adidt condition an archenteron. In other groups the 



