NOTES ON EMBRYOLOGY AND CLASSinCATION. 



407 



an " eruptiA^e " nor a " disruptive " character^ but to have 

 been rather " inruptive " — that is to say, the establish- 

 ment of the mouth as a permanent structure was accompanied 

 by an ingrowth of ectodermal cells^ no doubt very slight at 

 first, but afterwards attaining great size and importance as 

 the first portion of the alimentary tract. It is this ingrowth 

 which gives rise to what is often called " pharynx " in 

 Mollusca, Arthropoda, and Vermes. I have proposed ^ to 

 designate this ingrowth of the deron, the STOMODiEUM 

 (o-rojuoSatov, like irvKoZaiov, the road connected with a gate- 

 way), and similarly to call another ingrowth which accom- 

 panies the formation of the second orifice (the anus) of the 

 enteron, the proctodeum. The mouth and stomodseum 



Fr^j.S 



Tza. e 



Figs. 5 and 6. Delamination of Blastula of a Jelly-fish (after Fol.). 



He, Ectoderm. En, Entoderm (Enteron). Sch, Schizocoel. 



Fig. 7. tc, deron ; tn.^ enteron; Std., stomodseum ; Ftd.^ proctodseum. 



appear to have existed some time before an anal orifice was 

 developed, and the mouth must have functioned as it does in 

 living Zoophytes, both for the ingress of food to the enteron 

 and the rejection of undigested remnants. The development 

 of an anus and proctodseum may be conceived of as due to 

 the gradual establishment of an, at first, purely mechanical 

 rupture, as a permanent hereditary feature. In the recapitu- 



1 ' Quart. Journ. Mic. Sci.,' April, 1876. 



