414 PROFESSOR LANKESTER. 



hole character. Here we have a secondary accidental ac- 

 companiment of the invagination process — namely^ the 

 necessary orifice of invagination, rising suddenly to the 

 importance of a substitute for refracting media which were 

 differentiated in the tissues of the eye, before invagination 

 had supplanted delamiuatiou in its development. Just as 

 the orifice of invagination in the Cephalopod's eye is made 

 use of in certain Cephalopods, but not in all, so is the blasto- 

 pore made use of in certain invaginate Diblastulse, but not in 

 all, and not in all in the same way. 



In certain Prosobranchiate Gastropods, most carefully 

 studied by Bobretzky, the stomodseum, the deric (ectodermic) 

 ingrowth belonging to the mouth, occurs either at the spot 

 where the blastopoi'e has just closed, or before it Jias closed, 

 so that the oral ingrowth forms around the blastopore ; and 

 thus the blastopore does not close, though it is inaccurate to 

 to say that it becomes the mouth. The same process takes 

 place, in all probability, accoi'ding to Kowalewsky's observa- 

 tions, in the earthworm and in some zoophytes; also in 

 nematoid worms, according to Biitschli. These are the 

 cases which have led to the supposition that the blastopore is 

 the primitive mouth of Enterozoa, and these particular forms 

 are those which have been called archseostomatous. But it 

 will be observed that in these cases the normal mode of the 

 formation of the mouth is not departed from ; a stomodseum, 

 an ingrowth of ectodermal cells, takes place here as in the 

 ancestral delaminate type, which we have sketched above. 

 The mouth and stomodseum merely avail themselves, as it 

 were, of the blastopore soon about to close, and so a coinci- 

 dence is effected. 



In other cases — the Echinoderms, Paludina, among Pro- 

 sobranch Gastropods, and probably many other animals — it 

 is the anus, with its proctodseal ingrowth, which adapts itself 

 to the blastopore. In the Pulmonate Limuseus both anal and 

 oral growths develop on the site of the elongated blastopore. 

 By regarding all these cases as late and special adaptations 

 of the blastopore — itself a secondary structure concerned in 

 the mechanism of egg-cleavage, and not developed with a 

 specific function — we are able to explain, in a measure, the 

 very astounding fact that what becomes the mouth of a 

 whelk appears to become the anus of a water-snail. A very 

 slight mechanical variation of conditions may be conceded to 

 be sufficient to cause a small shifting in the position of the 

 unsignificant orifice of invagination, so as to bring it within 

 the predestined area of either stomodaeum or proctodaeum. 



Comparison of the applicability of the hypothesis of a primary 



