Frenzel’s Mesozoon Salinella. 473 
been transmitted from the Protozoa, and consequently fur- 
nishes a CONNECTING LINK, however small it be, between the 
two groups’”’*. We should certainly be confronted with 
great difficulties, although 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- 
gellata 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 passes in its life are also characterized by different modes 
of nutrition. In their different phases of life the Protozoa 
may resemble Amcebe, Flagellata, or Ciliata, or may pass 
through all three conditions (Catallacta of Heckel). The. 
same 1s 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 Volvoz, 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 
holophytic 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 holophytic or saprophytic nutrition; not only 
among closely allied genera are some holophytic (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 Huglena) ; 
nay, it is even possible for one and the same form in its pre- 
dominant phase of life to pass from the holophytic to the 
saprophytic mode of existence by losing its chlorophyll (e. g. 
Chlorogontum 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 (e. g. in septic Monads), though 
* F. Metschnikoff, ‘ Untersuchungen tiber die intrazellulare Verdauung 
bei wirbellosen Tieren,’ Wien, 1883, p. 2. 
