9 6 
F. Cavers. 
without a limiting cuticle, though frequently a test or shell is 
secreted, into which the protoplasm can be partly or entirely with¬ 
drawn ; the adult has no specialised organs of locomotion—flagella 
or cilia—but moves and ingests food by more or less temporary 
extrusions of the protoplasm termed pseudopodia, i.e., in the manner 
called amoeboid after the genus Amoeba. The Flagellata (Mastigo- 
phora of many zoological writers) have in the adult or “ vegetative ” 
or trophic phase one or more special contractile protoplasmic out¬ 
growths termed flagella which perform lashing whip-like movements; 
the protoplasm may be naked and amoeboid, but more often is 
limited by a cuticle. The Sporozoa, which are internal parasites of 
higher animals, absorbing food from the internal juices of their hosts 
and never having in the vegetative or trophic phases any organs of 
locomotion or ingestion of food, are either naked and amoeboid or 
covered by a cuticle ; as the name implies, they reproduce typically 
by producing numerous small spores, though spore formation occurs 
also in other Protozoa. The Infusoria, which have been termed 
Heterokaryota from the fact that the nucleus is apparently always 
divided into two parts specialised in function and differing also 
in size (meganucleus and micronucleus), move and capture food (at 
least in the young state) by means of cilia—contractile filamentous 
protoplasmic outgrowths differing from flagella in their larger 
number and perhaps also in the mode of contraction and movement. 
These four main groups have been variously combined into 
larger categories. For instance, Lankester (1885) divided the 
Protozoa into two main groups, the Gymnomyxa with naked 
protoplasm and indefinite form (Sarcodina), and the Corticata with 
the protoplasm limited by a firm membrane and hence with definite 
body-form ; but in the corticate groups there must be placed non- 
corticate amoeboid forms as Mastigamceba among the Flagellata or 
the malarial parasite among Sporozoa. Doflein (1911) made a 
twofold division into Ciliophora (Infusoria) with locomotion by cilia, 
and Plasmodroma with locomotion organs derived from protoplasmic 
processes, i.e., pseudopodia or flagella; but the distinction between 
flagella and cilia is not so sharp as to warrant such a division. 
Jackson (1888) suggested the union of the forms bearing flagella and 
cilia into one section, Plegepoda, and distinguished two other 
sections—Rhizopoda (Sarcodina), and Endoparasita (Sporozoa). 
The classification of Protozoa is generally admitted to be a task 
of great difficulty. The Infusoria and the Sporozoa, though the 
majority of their members are clearly marked off—in the former case 
by the heterokaryote nuclear apparatus and the peculiar form of 
conjugation involving the reorganisation of this apparatus, in the 
latter by the possession of at least two alternating modes of brood- 
formation, of which the first consists of aplanospores wherein is 
formed the second brood of sickle-shaped zoospores—cannot be 
sharply distinguished from other Protozoa when the lower forms of 
the two groups are taken into consideration ; while the Sarcodina 
and the Flagellata are not only difficult to separate from each other 
but include in each case, particularly in the former, a number of 
forms which are very variously distributed by different writers among 
the generally recognised groups (Proteomyxa, Rhizopoda, etc., see 
below)—groups that can be kept apart only by arbitrary distinctions. 
Since the Flagellata are a generalised group and may be said to 
