76 ORGANIC DEPENDENCE AND DISEASE 



14. Actinocrinus verrucosus Hall. 



15. A. multiramosus Wachsmuth and Springer. 



16. Acrocrinus shumardi Yandell. 



17. Poteriocrinus coccinus.^ 



18. Cromyocrinus simplex Trautschold.^ 



Fig. 64. Cromyocrinus simplex with attached snail of large size, its anterior 

 portion covering the anal aperture. Carboniferous limestone, Russia. 



In order to show that in the study of tliis adaptation be- 

 tween crinoid and snail we are not being misled by appear- 

 ances, let us look a moment to the mode of evolution of the 

 excurrent aperture in the crinoid and the effort made by 

 the crinoid host to protect itself against this disadvanta- 

 geous parasitism. 



1. In the crinoids of the Ordovician and Silurian, where 

 this adjustment first appears, there was only a smooth- 

 domed tegmen between the arms and, with these arms float- 

 ing expanded in search of food, the dome with its anal aper- 

 ture was a freely exposed surface, an open invitation to 

 such attachment as ensued. There were, however, plenty 

 of crinoids of that period provided with anal tubes, that is, 

 plated extensions of the tegmen or upper surface into pro- 

 boscis-like shapes, bearing the anus at their extremity, and 

 the purpose of this combination seems to have been to raise 

 the rejectamenta well above the mouth of the crinoid.^ 



On this matter Wachsmuth and Springer* have written : 



1 An uncertain species, thus spelled by Keyes but not otherwise known. 



2 From the Moscovian limestone of Russia. 



3 Among these genera are Diabolocrinus and Deocrinus of the Ordovician ; 

 Lampterocrinus, Siphonocrinus, Lyriocrinus, Eucalyptocrinus, Callicrinus and 

 Chicagocrinus of the Silurian. 



4 1897, "N. Amer. Crinoidea Camerata," p. 135. 



