56 KINGSLEY. [Vol. VII. 



There can be no question that delamination in these forms is 

 not a direct derivative from delamination in the Coelenterates. 

 It has rather arisen in the Arachnids and probably from a true 

 gastrulate type. The considerations which lead to this con- 

 clusion are these: — 



It is at least probable that the Arthropods have had an anne- 

 lidan ancestry, and in these latter forms delamination does not 

 occur. Hence we must either regard it as having been lost in 

 the segmented worms while it is retained in the Arachnids, or 

 we must consider it as of casnogenetic character in the latter 

 group. I believe that delamination, as it occurs in Limulus and 

 the Pycnogonids, may be traced back to an ancestral invaginate 

 condition ; in fact, all stages between a regular embolic gastrula 

 like that of Lucifer and the extreme delamination of the 

 Pycnogonids can be found in the Arthropod phylum, although 

 not in the Arachnids themselves. 



The series between Lucifer (Brooks, '82) with an arch- 

 enteric cavity of large size is easily traced through conditions 

 like those of Astacus and Palsemon, to that presented by Cran- 

 gon, where the invaginated entoderm is solid, but in which the 

 blastopore is still readily recognized. Crangon, on the other 

 hand, presents many similarities to Theridion (Morin, '87) and 

 the Japanese species of Agelena studied by Kishenouye. In the 

 forms just mentioned there is apparently ^ a time when every 

 nucleus has reached the surface and has participated in the for- 

 mation of the blastoderm, leaving the large central yolk in an 

 anneliate condition. Later, the blastoderm thus formed becomes 

 thickened by cell proliferation, and from the ridge thus formed 

 cells pass "into the yolk and become scattered without definite 

 arrangement through the entire yolk. These are the entoderm 

 cells" (Kishenouye, p. 62 ; cf. Kingsley, '86, p. no). 



Now in forms like Astacus, Palaemon, and Crangon the meso- 

 derm arises from the lips of the blastopore and from what may 

 be regarded as its forward continuation in the median line, and 

 from this fact we are justified in regarding the thickening which 

 in the Japanese Agelena and in Scorpio (Laurie) gives rise to 

 mesoderm and entoderm as an obsolescent blastopore homologous 

 with the actual open blastopore in the other forms mentioned. 



^ Kishenouye could not "detect any nucleus at all in the yolk, thus confirming 

 the views of Morin in opposition to Balfour's" ('90, p. 60). 



