xiv PREFACE. 



Vorderdarm, Mitteldarm, Hinterdarm, Kopfdarm, has caused me some 

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

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

 fact is that, whilst Ave 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, Uut the 

 English word "gut " is associated rather with the hinder than with the fore- 

 most portion of this tract. It will probably be found most convenient to 

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

 tube ;" and these 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 the same names in the Mollusca 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 endodermal 

 partly ectodermal in the Mollusca. 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 "Notes 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 by 

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

 corresponding passage leading from the anus I similarly propose to call 

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

 are most equally developed in the Arthropoda and some Mollusca. In 

 Vertebrata the stomodamm. is exceedingly small, if indeed its true homo- 

 logue exists at all (excepting in the Tunicata). The proctodeum 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 adult condition an archenteron. In other groups the 



