ORGANIC DEPENDENCE AND DISEASE 63 



in these early presentments see that the fixation which led 

 to recreancy was for a purpose distinctly advantageous to 

 ease of living; that the creature became consolidated and 

 set at a very early geological age. 



For the Lepad or goose barnacles we have very much 

 the same sort of evidence. There are much elongated 

 flat segmented barnacles in the Ordovician known as Lepi- 

 docoleus. Others of this type occur throughout the Silu- 

 rian and Devonian (Plumulites, Turrilepas, Strobilepis^) in 

 which the elongated shape and the regular fore-and-aft 

 ranges of plates are retained, but the earlier form is much 

 the simpler, as it has but two such ranges, one on each side 

 and both symmetrical. Attachment in all of these is by the 

 base, that is, by one end of the shell ; and we have expressed 

 the belief that the whole stock to which these belong and 

 which is represented by the existing Lepads arose from a 

 like phyllopod ancestor to the Balanidae but through fixa- 

 tion or cementation by the head, rather than the back; a 

 fixation which gave a similarly favorable exposure of the 

 feeding and breathing appendages, but also induced greater 

 exposure to lateral stresses, which has resulted in a more 

 numerous development of sutures and valves. This attach- 

 ment, and consequently the career of the goose barnacles 

 would seem to date from approximately the same geologi- 

 cal age as that of the Balanidae and, as we have said, to 

 have come out of the same free and independent crustacean 

 stock. 



1 These have been commonly accepted as Cirripedia and while we are grant- 

 ing that the probabilities seem in favor of this interpretation, there are features 

 of structure which suggest the scaled annelids. This comparison has not been 

 closely carried through. 



