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112 ORGANIZATION OF THE INFUSORIA.Polyzoa that may be gathered at low-water mark or dredged from thesea-bottom. These in almost all instances produce an abundant harvestof Zoothamnium alternans and other attached Vorticellidas, Acinetidae, andFlagellata, being in this respect far more prolific than the neighbouring seaweeds,though certain of the finer divided varieties of these will be foundto yield their characteristic types. Many free-swimming species may beof the smaller rock-pools, and moresecured by a careful surface-skimmingespecially such as are invaded by the sea only during the spring tides andare left undisturbed through the intervening intervals. As in pond collecting,the innumerable small Crustacea, Annelids, and other animal organismswith which the sea abounds, should be carefully examined for parasiticforms. For the capture of the less familiar pelagic types, including representativesof the families of the Noctilucidae, Peridiniidae, Dictyocystidae, andTintinnidae, the use of a boat and muslin towing-net must be resorted to.Concerning the geographical distribution of the Infusoria, we cannotbe said at present to possess anything approaching a comprehensive knowledge.The infusorial fauna of but few countries or even districts hasas yet been thoroughly investigated, while whole continents remain ofwhich it may be said that absolutely nothing whatever is known. Fromsuch data as are at present available, nevertheless, and as might be anticipatedwith reference to organisms occupying so humble a position in theorganic scale, it is made manifest that generic, and in many cases evenspecific, forms exhibit a most diffuse and practically cosmopolitan area ofdistribution. Thus, a number of types belonging to identical or indistinguishablespecific forms are found to occur on the European continentiin England, North America, and the neighbourhood of Bombay. Arctic,temperate, and tropic climes yield, so far as at present explored,no morewidely divergent types than are to be encountered within adjacent districts.Even the marine species as a rule exhibit no clear and well-defined demarcation,for the most part including the same generic and in not a fewinstances specific sorts identical with those obtained from fresh water.Of the dissemination of Infusoria in time, or in other words their geologicaldistribution, there is, as a necessary consequence of their mostly softand perishable nature, scarcely a trace preserved. The hard coverings orloricae of a few species have, however, been recognized by Ehrenberg inthe Cretaceous deposits, and referred by him to the genera Ch