372 WILLIAM PATTEN. 



But there are certain structures in Arthropods which may 

 represent remnants of gastrulas. For example, if the mouth 

 and oesophagus of Arthropods is primitive — and there is no 

 reason to suppose it is secondarily acquired, — then we must 

 look for primitive eudoderm at its inner end. I have figured 

 in " Eyes of Acilius/^ at the very anterior end of the embryo, 

 a great sac of endoderm cells which probably arise by in- 

 vagination, although the process was not directly observed. 

 The sac, which soon opens outward by the oesophagus, after- 

 wards becomes solid, and finally is converted into two longi- 

 tudinal bands, one on either side, extending backwards 

 towards the middle of the body, where they become con- 

 tinuous with similar bands extending forwards from the 

 posterior end of the embryo. 



There are several other cases where great vesicular cells 

 appear at the inner end of the oesophagus, and they probably 

 are of a similar nature to those just described. 



In Limulus there is a great lump of endoderm-like cells at 

 the inner end of the oesophagus ; they grow fainter, and are 

 quickly absorbed without forming either yolk-cells or any part 

 of the definite endoderm. 



These endoderm-cells are the only structure in Arthropods 

 which I can see any reason for regarding as remnants of a 

 gastrula. This evidence consists solely in their position at the 

 inner end of the oesophagus, and their speedy absorption in 

 Limulus, and possibly other Arthropods. But even this 

 evidence is of no great weight, since in Doryphora — according 

 to Wheeler, Avhose preparations I have had the privilege of ex- 

 amining — a great mass of cells arising from the posterior end 

 of the body is absorbed in very much the same manner as the 

 oesophageal ones in Limulus. 



The history of the layers in Scorpio and Limulus is com- 

 plicated, and I have not yet obtained satisfactory results ; 

 much of the endoderm and mesoderm is produced from the 

 cluster of cells at the posterior end of the body, but there are 

 no such distinct endodermic bands as in insects. It is 

 probable that some of the endoderm, and perhaps meso- 



