! I0 ORGANIZATION OF THE INFUSORIA. 



invariably attached to other living organisms, are not maintained at the 

 expense of the essential juices of these latter, but simply occupy with 

 respect to them the position of co-lodgers or messmates. The very 

 appropriate title of "commensals" has been recently employed by Prof. 

 P. van Beneden to distinguish those organisms among the higher Metazoic 

 series which pass a similar co-associated, non-hurtful, and often mutually 

 dependent existence, and that of " commensalism " for the distinction of 

 the peculiar pseudo-parasitic life habits which they exhibit. As might be 

 anticipated, the phenomenon of commensalism is exhibited among the 

 Infusoria chiefly by those types that lead a fixed or sedentary existence, 

 and is notably conspicuous among the Peritrichous Vorticellidae. From 

 these may be selected as examples various species of the several genera 

 Epistylis, Zoothamnium, Opercularia, Cothurnia, Scyphidia, and Spirochona, 

 a very large number of which are found in company only with certain 

 species of aquatic Insecta, Crustacea, or Mollusca. 



As free or unattached messmates or commensals, reference may be more 

 especially made to the family of the Urceolariidae, including Trichodina 

 pediculus, notable for its intimate relationship with the fresh-water Hydra, 

 and which position it shares with the Hypotrichous form Kerona polypornui. 

 Other animalcules of the same family, exhibiting closely corresponding 

 habits, are rep'resented by the two genera Urceolaria and Trichodinopsis , 

 while a near ally, Licnophora, has been found as yet as a commensal only of 

 certain marine Planarians. The genus Ophryodendron, among the Tenta- 

 culifera, furnishes in the two species O. sertularia and O. multicapitata 

 marked examples of pseudo-parasitic life, the former being a common 

 guest of both Sertularian zoophytes and the little hairy crab Porcellana 

 platycheles, while the latter, recently discovered by the author, has as yet 

 been found attached only to the limbs and carapace of a species of sessile- 

 eyed crustacean. Descending to the division of the Flagellata, commen- 

 salism, so far as is at present known, would appear to be most abundantly 

 represented among the mostly sedentary collar-bearing section of the 

 Choano-Flagellata, where it is further noteworthy that the majority are 

 found attached, as commensals, to the peduncles or loricae of other higher 

 Infusoria. In this manner Salpingoeca minuta is found in society with the 

 flagellate type Dinobryon sertularia, while Salpingoeca convallaria grows on 

 Epistylis anastatica, and Monosiga Steinii on Vorticella convallaria and 

 Epistylis plicatilis, itself a commensal of the common pond-snail. 



While the foregoing summary of some of the more abnormal areas of 

 distribution of the Infusoria subserves the purpose of indicating to the 

 student and collector of this group of beings the localities in which to seek 

 with success for certain specific or generic types, a brief space devoted to 

 an enumeration of the most favoured habitats of the non-aberrant types 

 will probably be welcome. In this direction it need scarcely be indicated 

 that weedy ponds, and slowly running water, containing an abundance of 

 aquatic plants, present both the most accessible and most remunerative 



