ORGANIC DEPENDENCE AND DISEASE 77 



'^As a rule, a tube occurs more frequently among species in 

 which the arms form a continuous series around the calyx, 

 while species in which they are arranged in clusters often 

 have a simple opening. ' ' And it also appears that this tube 

 was of convenient rather than essential function. The ani- 

 mal could live without it when it was broken off or ob- 

 structed, and easily made a new orifice when necessity re- 

 quired. 



2. It appears, however, that early attachment may have 

 prevented the development of the anal tube. This would, 

 indeed, seem to be indicated by the specimen of Cratero- 

 crinus ruedemanni here shown, where the attachment be- 

 ginning very early prevented the development of an anal 

 tube which would have been normal to this type of struc- 

 ture, or at least is normal to all other members of the fam- 

 ily to which this belongs — the Dolatocrinidae. 



3. In general, crinoids which have been found to bear 

 parasitic gastropods have no anal tube but only a slight 

 protuberance. Exceptions to this statement are rare and of 

 uncertain value. Actinocrinus verrucosus Hall has an anal 

 tube but a specimen is reported by Keyes, with an attached 

 snail,^ In this crinoid, however, it appears that the tube 

 has been broken off and an aperture developed on the dome. 

 Wachsmuth and Springer refer to Actinocrinus multiramo- 

 sus in which this anal tube is retained and a gastropod is 

 attached to the tegmen below. ^ 



4. The parasitic condition so rampant in these early 

 Carboniferous ages disappeared entirely so far as present 

 records indicate, with the disappearance of the fauna from 

 the various shallow seas of the world. There are instances 

 of the continuation of some of these parasitized genera of 

 the Mississippian into the marine faunas of the Coal Meas- 

 ures (that is, Aorocrinus) but in none is the condition of 



1 Keyes, 1888, pp. 235, 237. 



2 1897, "N. Amer. Crinoidea Camerata," p. 566. 



