FrenzeVs Mesozoon Salinella. 473 



been transmitted from tlic Protozoa, and consequently fur- 

 nishes a C()NN1':CT1N(! LINK, liowevcr small it be, between the 

 two groups " *. We should certainly be confronted with 

 great difficulties, althougli not on the ground of nutrition, if 

 we would construct Metazoa from Infusorian-like unicellular 

 animals, such as the "larva" of Salinella. But we must 

 not select precisely the most improbable possibility. The 

 very earliest Metazoa are, as is generally agreed, to be 

 derived from Flagellate-like creatures ; and among the Fla- 

 gcilata it is really only a question of the stage of develop- 

 ment whether the digestion of an animal is extra- or intra- 

 cellular ; the different conditions of form through which the 

 cell jiasses in its life are also characterized by different modes 

 of nutrition. In their different phases of life the Protozoa 

 may resemble Amocbre, Flagcllata, or Ciliata, or may pass 

 tiirough all three conditions (Catallacta of Ilajckel). The 

 same is also true of a large number of cells in the body of 

 the Metazoon. Should we actually wish to consider holo- 

 phytic Flagellates as ancestral forms of the Metazoa, in which 

 connexion a very pretty transition is realized by Volvox^ it is 

 easy to believe that, so soon as a communication between 

 the central cavity and the exterior became established, or in 

 some other way a gastral cavity arose, the cells gave up their 

 holophytie mode of life, to pass for the first time to an extra- 

 cellular method of digestion. We even find that real highly 

 organized plants are also capable of digestion on occasions, 

 and indeed of extra-cellular digestion, like the insectivorous 

 plants. In criticizing the relationships of the Flagellates it 

 is of no importance whatever whether a particular form 

 possesses holophytie or saprophytic nutrition ; not only 

 among closely allied genera are some holophytie (e. g. Chla- 

 mydomonas and Cryptomonas) and the others [Polytoma and 

 Chilomonas) saprophytic, but the mode of nutrition also 

 changes within the genus (the various species of Euglena) ; 

 nay, it is even possible for one and the same form in its pre- 

 dominant phase of life to pass from the holophytie to the 

 saprophytic mode of existence by losing its chlorophyll (e. g. 

 Chlorogonium and Carteria). The transition is, however, 

 very easy between saprophytic forms, and therefore such as 

 really do not digest, and those which are capable of digestion, 

 and as a matter of fact the digestion is for the most part 

 intra-cellular, in correspondence with the amoeboid form 

 which has been assumed (<?. g. in septic Monads), though 



* E. Metschnikoff, ' Uutersuchuugen iiber die iutrazellulare Verdauimg 

 bei wirbelloseu Tieren,' Wieu, 1883, p. 2. 



