36 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 72 



preying upon the crinoids, though this mode of hfe has not induced 

 any special modification of their structure. 



There is a curious and interesting correspondence between the 

 relations of the fixed marine organisms (including the crinoids) and 

 their parasites and commensals and those between parasitic and epi- 

 phytic flowering plants and their hosts. The barnacles, most hydroids, 

 polyzoans, etc., correspond very closely to the epiphytic plants, espe- 

 cially 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 microplankton in or 

 approaching the stomach of the host. On land most animals are 

 parasitized by animals of an inferior organization; but among the 

 fixed marine animals the parasites for the most part belong to a 

 phylum with a superior organization and sometimes even to the same 

 phylum (coelenterates parasitic on coelenterates, crustaceans parasitic 

 on crustaceans, ophiurans parasitic on crinoids, etc.). The relations 

 between the fixed marine animals and their parasites are thus more 

 nearly the same as those between parasitic flowering plants and their 

 hosts. On land the various animal groups are definitely parasitic or 

 nonparasitic ; but many plant families, such as the Scrophulariaceae, 

 Santalaceae, etc., and even many single genera, such as Pedicularis, 

 Melampyrum, Gerardia, etc., include both parasitic and nonparasitic 

 species, just as do many families and genera, such as Synalpheiis, 

 Periclimenes, etc., occurring with the fixed marine animals. 



The three types of parasites which are of especial interest are the 

 gasteropods {Stilifer, Stylina, Sahinclla and Melanella), Enterog- 

 nathus and the myzostomes (figs. 57, 58). 



The family Melanellidse to which Stilifer, Stylina, Sahinella and 

 Melanella belong includes species showing all gradations between 

 free-living nonparasitic types and shell-less parasites living entirely 

 within the body of the host. As parasites the Melanellidae occur only 

 upon the echinoderms, in which group, however, they are found on 

 species of all the classes. Most of the parasitic forms, including all 

 of those occurring on the crinoids, are characterized by extraordinarily 

 delicate shells. Some of the species are permanently fixed in one 

 position on the body of the host, but others, including all those found 

 upon the crinoids, appear to move about and to bore into dififerent 

 parts of the host. It is not a little curious that, apart from Melanella 

 capcnsis and Stylina comatidicola, all the species parasitic on the 



