76 
ORGANIC DEPENDENCE AND DISEASE 
14. Actinocrinus verrucosus Hall. 
15. A. multiramosus Wachsmuth and Springer. 
16. Acrocriuus shumardi Yandell. 
17. Poteriocriuus coccinus. 1 
18. Cromyocrinus simplex Trautschold. 2 
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 this 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. 3 
On this matter Wachsmuth and Springer 4 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. 
