112 ORGANIZATION OF THE INFUSORIA. 



Polyzoa that may be gathered at low-water mark or dredged from the 

 sea-bottom. These in almost all instances produce an abundant harvest 

 of Zoothamnium alternans and other attached Vorticellidas, Acinetidae, and 

 Flagellata, being in this respect far more prolific than the neighbouring sea- 

 weeds, though certain of the finer divided varieties of these will be found 

 to yield their characteristic types. Many free-swimming species may be 

 secured by a careful surface-skimming of the smaller rock-pools, and more 

 especially such as are invaded by the sea only during the spring tides and 

 are left undisturbed through the intervening intervals. As in pond collect- 

 ing, the innumerable small Crustacea, Annelids, and other animal organisms 

 with which the sea abounds, should be carefully examined for parasitic 

 forms. For the capture of the less familiar pelagic types, including repre- 

 sentatives of the families of the Noctilucidae, Peridiniidae, Dictyocystidae, and 

 Tintinnidae, the use of a boat and muslin towing-net must be resorted to. 



Concerning the geographical distribution of the Infusoria, we cannot 

 be said at present to possess anything approaching a comprehensive know- 

 ledge. The infusorial fauna of but few countries or even districts has 

 as yet been thoroughly investigated, while whole continents remain of 

 which it may be said that absolutely nothing whatever is known. From 

 such data as are at present available, nevertheless, and as might be antici- 

 pated with reference to organisms occupying so humble a position in the 

 organic scale, it is made manifest that generic, and in many cases even 

 specific, forms exhibit a most diffuse and practically cosmopolitan area of 

 distribution. Thus, a number of types belonging to identical or indis- 

 tinguishable specific forms are found to occur on the European continenti 

 in England, North America, and the neighbourhood of Bombay. Arctic, 

 temperate, and tropic climes yield, so far as at present explored, no more 

 widely divergent types than are to be encountered within adjacent districts. 

 Even the marine species as a rule exhibit no clear and well-defined demar- 

 cation, for the most part including the same generic and in not a few 

 instances specific sorts identical with those obtained from fresh water. 



Of the dissemination of Infusoria in time, or in other words their geolo- 

 gical distribution, there is, as a necessary consequence of their mostly soft 

 and perishable nature, scarcely a trace preserved. The hard coverings or 

 loricae of a few species have, however, been recognized by Ehrenberg in 

 the Cretaceous deposits, and referred by him to the genera Ch<ztotypJila and 

 Ceratium. The siliceous loricae of the Peritrichous genus Dictyocysta occur 

 also in company with the pelagic Polycistina of the Tertiary formations, 

 and there can be but little doubt that the Infusoria in their simplest 

 condition represent one, if not the most ancient, of the stock-forms of the 

 organic series. Accepting, in fact, the Spongida as representatives of 

 the Choano-Flagellata, a subject-matter which is fully discussed in a suc- 

 ceeding chapter, the Flagellate section of the infusorial series is distinctly 

 traceable through every marine deposit from the present day back to the 

 Cambrian and even the Laurentian section of the Palaeozoic epoch. 



