152 MARINE BIOLOGICAL LABORATORY. 



opaque area on the surface — the primitive cumulus ; 

 beneath this the first traces of the mesoderm are found, 

 and further, this is the point where the stomodaeum 

 soon invaginates. We have seen an exactly similar 

 series of changes in the embryos of the Pycnogonids. 

 We also find body cavities lying at the bases of the 

 legs of the embryo spider, in an exactly similar position 

 to those which we have seen in the sea-spiders. The 

 digestive pouches found in both the adult and in the 

 embryos of the Pycnogonids, which contain yolk in 

 the young, furnish us with another point of comparison 

 with the Arachnids. In the latter, we have these for 

 a time in the developing young ; and, moreover, in the 

 false-scorpion they contain yolk from the mesenteron. 



Lastly, I must call attention to the structure, inner- 

 vation, and origin of the first pair of appendages. 

 These are chelate, as in many Arachnids, and the free 

 movable joint of the claw moves outwards and down- 

 wards as in the Arachnids generally (and not upwards 

 and outwards as do the chelae of the Crustacea). Again, 

 these appendages are innervated in the sea-spiders from 

 the brain, and finally in the embryo the appendages 

 are seen to arise at the sides of the stomodaeum, and 

 subsequently move forward and dorsal to it. In both 

 of these last characteristics we can compare the two 

 groups directly together. 



So much for the embryology : I leave these com- 

 parisons to speak for themselves. 



And now let us turn to the adult structures of the 

 two groups under comparison. 



I do not believe any general definition of the group 

 of Arthropods can be made, which, if based on funda- 



