No. 2.] THE EMBRYOLOGY OF LIMULUS. 247 



There is one point of resemblance between the Arachnids 

 and the Hexapods which may have no inconsiderable weight. 

 In the Scorpions as in the Hexapods, the embryo develops 

 those as yet unexplained foetal membranes which so closely 

 simulate those of the higher vertebrates. It may be that 

 here, as in other places, we have similar but not identical 

 organs. The accounts of their development in the Arachnids 

 by Metschnikoff, Kowalevsky and Schulgin, and Laurie differ 

 considerably, and until we know something of the ancestry 

 and real meaning of the structures which are united under 

 this head we cannot be certain of the taxonomic value to be 

 placed upon them. It may be noted here that the structures 

 described by Bruce ('87) as occurring in the spiders are in all 

 probability not amnion and serosa, but either the invaginations 

 in connection with the brain or the inpushing to form the 

 median eye. 



The Classification of the Arthropoda, 



As a result of my studies it would seem as if the Arthropoda 

 must be divided in some such manner as that here given : — 



Phylum Arthropoda. 



Sub-Phylum Branchiata. 

 Class Crustacea. 

 Class Acerata. 



Sub-Class Giijantostraca. 

 Sub-Class Arachnida. ■"■ 

 Sub-Phylum Insecta. 

 Class Hexapoda. 

 Class Chilopoda, 

 Sub-Phylum Diplopoda (Chilognatha). 



IncertcB Scdcs. 



Pauropoda. 



Pycnogonida. 



Trilobitas. 



Tardigrada. 



Malacopoda. 



^ The attempt by Ilaller ('81) to separate the Acarina a^ a distinct class hardly 

 seems warranted. 



