164 MARINE BIOLOGICAL LABORATORY. 



furnish sufficiently good grounds for rejecting such a 

 theory as exceedingly improbable. 



First, I must insist upon the fact that those char- 

 acteristics which the Pantopod-larva has in common 

 with the Trochophore are only those features which 

 any two Arthropods have as common characteristics of 

 the first three segments ; or, if we go further and include 

 the Annelids, we may say which any two Articulates 

 possess as common characteristics. They are these: 

 a brain and ventral chord, united by commissures 

 around the oesophagus; serial appendages correspond- 

 ing in number to the number of segments ; a digestive 

 tract with stomodaeum, mesenteron ; and — and here 

 lies the difficulty : not a trace of proctodaeum does 

 the Pantopod-larva possess, while both Nauplius and 

 Trochophore have the digestive tract open posteriorly. 

 In the young Pantopod-larva we have, so far as it goes, 

 a fully formed and presumably functional digestive 

 apparatus, and no reasonable account can, I believe, 

 be given to explain how this posterior opening could 

 have become lost in the transition of Trochophore into 

 Pantopod-larva as Dohrn has supposed. If it be urged 

 that in the young form the anus may have become func- 

 tionless, and hence not developed, it will not mend 

 matters, since we have every reason to believe that in 

 the adult Pycnogonid the proctodaeum is to a large 

 extent useless.^ 



I think these facts must prejudice us strongly against 

 the position taken by Dohrn ; but let us go further and 

 examine into the other features of the Pantopod-larva. 



1 It has been suggested that it may serve for purposes of respiration, but 

 no evidence for this is forthcoming. 



