/ 



outline and remains so in the adult. vSchimkewitsch describes a similar 

 triangular invagination in the Spiders, and compares it directly to that of 

 the Pycnogonids. 



The full meaning of the ventral organs I cannot discuss now. I have 

 compared them to similar organs of Peripatus. It may be that in this 

 respect the Pycnogonids show a very primitive structure, common to them 

 and to Peripatus, and if so, traces ought to occur most probably in other 

 Arachnids. 



The absence of brain invaginations would be a more weighty objection 

 against the relationship of the two groups, and really the only good objec- 

 tion I know from the embryology. 



The openings of the reproductive organs of the adult on the legs cannot 

 be fairly urged against my comparison, for we have so far no explanation 

 of the meaning ; and on the other hand this gives little better foundation for 

 a relationship with the Annelids. 



All the above comparisons are not of equal weight, and some may be 

 wrong ; but taken all in all, I must appeal to them to bear out the hypoth- 

 esis of the relationship of the Pycnogonids to the Arachnids. 



Baltimore, March 15, 1890. 



