MONOGRAPH OF THK EXISTING CBINOEDS. 615 



III. Nonparasitic commensals: Animals which, while usually, or commonly, 

 found living upon or among the crinoids, lead an entirely independent existence, 

 and for the most part are found living under similar relations with other organisms : 



Hippolyte, sp. Scalpeltum, spp. 



Scalpellum album. Verruca nitida. 



Scalpellum balanoides. Verruca, spp. 



Scalpellum diceratum. Pachylasma crinoidophilum. 



Scalpellum. gonionotum. GemeUaria loricata. 



Scalpellum pentacrinarum. Loxosomella amtedonis. 



Scalpellum weltnerianum. Rhabdopleura mirabilis. 



Parasitic hydroid. 



IV. Casual associates : Animals which normally occur hiding among, crawling 

 over, or attached to other usually arborescent organisms, from which they may 

 or may not derive nourishment, or which normally occur attached to any avail- 

 able support, and which occasionally stray among or upon, or attach themselves 

 to, the crinoids but remain otherwise entirely independent of them : 



Small fishes. Other ophiurans. 



Many crustaceans. Certain pelecypods (Avicula, etc.). 



Certain ascidians. Serpulidae. 



Astrogomphus vallatus. Calycella syringa. 



Aster opor pa annulata. Stegopoma fastigatum. 



Sigsbeia murrhina. Cuspidella, sp. 



Ophiacantha, spp. Lafoea fruticosa. 



OpJiiolebes scorteus. Other hydroids. 



Ophiocoma, sp. Certain sponges. 



Ophiopholls mirabilis. Truncatulina lobatula. 



fOphiactis, sp. Polytrema miniaceum. 



Ophiomusium, sp. Other foraminifera. 



As in the case of the other arborescent marine types, and in general among 

 the animals that live by filtering the microplankton from the sea water, the 

 crinoids are chiefly subject to indirect parasitism; that is to say, the creatures 

 depending upon them for their existence appropriate the food particles which the 

 crinoids have collected in the ambulacral grooves, or even which they have 

 swallowed, instead of consuming the tissues or body fluids directly. Of the 

 animals which derive a part or all of their nutriment from the body, or from the 

 efforts, of the crinoids, about 10 per cent are directly parasitic, and about 90 per 

 cent are indirectly parasitic, in varying degrees. 



There is a curious and interesting correspondence between the relations of the 

 fixed marine organisms (including the crinoids) and their parasites and com- 

 mensals and those between parasitic and epiphytic plants and their hosts. The 

 barnacles, most hydroids, polyzoans, etc., correspond very closely to the epiphytic 

 plants, especially those of the families Orchidacese and Bromeliacese. Rhabdo- 

 pleura and certain hydroids are quite vine-like in habit, ascending crinoid stems 

 as vines do the trunks of trees. Most parasitic plants appropriate the unelaborated 

 sap of the host and convert it to their own ends ; most parasites of the fixed marine 

 organisms in the same way appropriate the concentrated but undigested micro- 

 plankton in or approaching the stomach of the host. On land most animals are 

 142140 21 Bull. 82 41 



