MONOGRAPH OF THE EXISTING CRINOIDS. 619 



of the crinoids from intermediate and great depths, which enable the animals to 

 convert the ambulacral grooves into closed tubes, and more or less completely to 

 close the mouth, and prevents the appropriation of the food particles by such types 

 as Synalpheus, Periclimen-es, Pontoniopsis, Galathea, Anilocra, Cirolana, Ophi- 

 actis, Ophiomasa, Ophiocethiops, Ophiophthirius, Ophiosphcera, or Polynoe. 



The myzostomes have developed means of avoiding this difficulty, such as 

 boring into the animal by the side of the ambulacral groove and causing the forma- 

 tion of a cyst in which they live, while the gasteropods bore into the body of the 

 crinoid anywhere and feed upon the tissues. 



The larger commensals living on the crinoids are usually striped or banded, 

 and resemble them more or less closely in color, though in many cases the closely 

 related noncommensal species are quite plain. This may or may not be the case 

 with the myzostomes. 



In regard to the parasites and commensals of the comatulids there is one 

 curious feature which stands out very prominently the majority of the records, 

 especially of the larger and more vigorous types, are based upon species of the 

 family Comasteridse. 



The large species belonging to the family Comasteridse are probably the most 

 highly specialized of all the comatulids. They almost, or even quite, completely 

 lack an adambulacral skeleton; the cirri tend to disappear; the central organ is 

 reduced to a minimum ; the radials are reduced to a minimum ; the arms in length, 

 size, or number are developed to the maximum, and are extraordinarily tough ; the 

 pinnules bear peculiar and highly specialized terminal combs; the digestive tube 

 makes about four complete turns instead of only slightly more than one ; and the 

 average size is very large. 



In the Comasteridae, therefore, we have what appears to be an overspecialized 

 group consisting chiefly of giants which are relatively helpless; their ambulacral 

 grooves can not be closed; they can not, to rid themselves of parasites, readily 

 cast off their arms or their disks; and they can not swim away from an infested 

 locality. As a natural result they are attracting to themselves as parasites repre- 

 sentatives of many types Avhich are quick to adapt themselves to any advantageous 

 new conditions. If this process is continued much further it will mean the disap- 

 pearance of the dominant comatulids of the Indo-Pacific and north Australian 

 reefs, and the reduction of the Comasteridae to small and generalized primitive 

 types resembling such genera as Comatonia, Comatilia, and Comatulides. 



The more or less sudden extinction of highly specialized giants has occurred 

 over and over again in past ages, and the incipient disappearance at the present 

 time of such types as the elephants, camels, bison, rhinoceroses, tapirs, and whales, 

 as well as the giant tortoises, sea turtles, and giant lizards, is patently evident. 



In these cases man is the aggressor, and the process has in recent years been 

 retarded by the enforcement of restrictive laws. But with the Comasteridae we 

 see the same thing working out in nature, probably just as it did in past ages, 

 through the convergence of a progressively increasing number of parasitic types 

 upon a relatively helpless group. 



