50 REVIEWS—SCIENTIFIC MANUALS, ETC. 
giving characters to the remaining sub-kingdoms,—and so in the 
animal kingdom, the development and disposition of the nervous 
system characterise the four higher divisions, whilst Protozoa are 
animals consisting of an animated jelly, (Sarcodium) with little dif- 
ferentiation of parts and no perceptible nervous system. We are 
aware indeed that there may possibly not be one of the sub-kingdoms, 
certainly none excepting the highest, in which there are not instances, 
where no nervous system can be demonstrated ; but in all such in- 
stances there is a manifest conformity to a type of structure, which 
directs our judgment as to the position of the object, whilst in Pro- 
tozoa, wherever we have a tolerable acquaintance with the lite history 
of the creature we recognise not only the absence of the character- 
istics of another sub-kingdom, but the presence of certain features 
properly belonging to that we are considering. If we have materials 
in our hands which really justify us in establishing a sub-kingdom of 
Protozoa, they can hardly fail to suggest some opinion as to the 
mode of sub-dividing it. If groups of creatures have been examined 
and intelligibly described, the question of their relation to other known 
groups, and the comparative importance of their distinctive marks 
will arise, and should be solved to the best of our ability. 
It seems to us, that the possession of a mouth, and consequently 
of an alimentary sac, with a somewhat definite figure, and an outer 
covering, differing in some degree from the mass of the body, char- 
acterise Infusoria (in the now received limited sense,) as the highest 
class of Protozoa. From them, Hhizopoda are distinguished, by 
having no difference, so far as is known, or only a slight difference in 
certain parts, in their external covering from the mass of their bodies, 
and by their power of protruding portions of their substance, in the 
form denominated Pseudopodia. Possibly the naked Rhizopoda, the 
Arcellina, the Foraminifera, and the Polycystina may be so many 
good orders in this class. Thalassicollida may be nearer akin to 
Sponges: of Gregarinida, nothing can as yet be satisfactorily decided, 
until a full history of at least some species, removes the doubts which — 
at present are unavoidable respecting their nature. 
Sponges for which we may adopt the name of Amorphozoa, form a 
third distinct class. Since no protrusion of pseudopodia is attributed 
to Thalassicollida and in some of them at least, cellaform bodies, 
seeming to contain germs are surrounded by spicules, not unlike the 
peculiar ovarian spicules of some sponges; we may perhaps regard 
