158 MARINE BIOLOGICAL LABORATORY. 



There is another problem, which must arise in any 

 discussion of the affinities of the Pycnogonicls, and 

 which may be treated as a corollary to what has gone 

 before. I refer to the presence within the group of a 

 free-living larval form — the Pantopod-larva which we 

 must believe typically to belong to all the species of the 

 order.^ Was the ancestral form from which the group 

 has arisen similar to this Pantopod-larva, hence its pres- 

 ence as a stage of development to-day ; or is it a purely 

 secondary larval form, an interpolated stage, therefore, 

 unimportant from a phylogenetic point of view ? 



Before attacking the problem directly, let us under- 

 stand clearly the possible conditions under which such 

 a larval form, or any larval form, may occur within a 

 group of animals. There are at least, two, and perhaps 

 three, such conditions generally recognized. We have 

 first what is known as a primary larval form, by which 

 we mean the young form represents approximately the 

 structure possessed by the ancestor from which the 

 group has arisen. In Fig. 4 this is shown diagrammati- 

 cally by A. The X marks the place at which a species x 

 expanded into a group, and after the group has been 

 evolved each young form in that group first reaches X, 

 and then each in turn develops the peculiar features 

 of its particular family, genus, species. 



Again we may find all the animals of a group possess- 

 ing a common larval form, but which need not neces- 

 sarily ever have been an ancestor of the group, but may 

 have been interpolated quite early in the history of the 



^ Such exceptions, as Pallene for instance, show an abbreviation of this 

 development, and at one time they or their ancestors must have had a 

 free-larval form. 



