232 
TOPULAR SCIENCE REVIEW. 
relationship between them and the Rhizopods than now exists ; 
as 6 survivals,’ indeed, from a remote and very early stage in the 
evolution of the Order. They indicate the line of development 
along which the Hydroida have passed, and throw a light on 
the cradle of the race. If we may accept Professor Allman’s 
ingenious interpretation of the Graptolites, a tribe of Silurian 
fossils, which has been bandied about from one division of the 
animal kingdom to another, but has now settled down amongst 
the Hydroida, we shall have still more direct testimony to the 
same effect. The Graptolite consists of a tubular stem, carrying 
a series of tubular offsets on one side of it, or on two opposite 
sides, and inclosing a solid rod or axis. It bears a sufficiently 
close resemblance to the Hydroida in general aspect and 
arrangement of parts to justify its association with them, not- 
withstanding certain points of divergence. The tubular offsets 
have been usually regarded as equivalent to the calycles in the 
recent species ; but Allman, from an examination of their struc- 
ture, has come to the conclusion that they represent sarcothecse 
rather than the dwellings of the polypites. To him the Grapto- 
lite is a Plumularian minus calycles and (probably) polypites ; 
a Plumularian hardly distinguishable from a composite Rhizopod, 
reduced to a level with this humble type in its ways of life, 
and dependent for its nutriment on the movements of its 
protoplasm. The Graptolite, we may say, according to this 
view of it, indicates the transition from the Protozoan to the 
Coelenterate structure. It can hardly be admitted that there is 
as yet an adequate foundation for this ingenious theory,* but 
whether we accept or not the conjecture of the accomplished 
biologist, I should be inclined to hold that the mere presence of 
the amoeboid bodies in organic connection with the Plumu- 
larian, side by side with the polypites, as associated zooids in one 
and the same colony, is in itself no obscure indication of a 
genealogical relationship between Rhizopod and Coelenterate 
and of their primitive affinity. 
* Allman grounds Lis view of the morphology of the Graptolites, mainly 
on the peculiar structure of their supposed calycles, which differ essentially, 
he thinks, from those of all recent liydroids. Their cavity is uninterruptedly 
continuous with that of the main stem, and there is no constriction or par- 
tition of any kind at the base ; and in this they agree perfectly with the 
sarcothecse, whereas, in the living hydroids, the calycle is marked off, more 
or less distinctly, as a proper chamber, by a constriction or imperfect dia- 
phragm. But it may be remarked on this, that the latter character is by no 
means universal. In the genus Sakicia, for instance, the calycles are not 
separated from the stem by any constriction, and the polypites when contracted 
can withdraw themselves wholly from them into the tube of the stem. In the 
genus Cuspidella there is also an absence of all constriction at the base of 
the calycles ; so that this characteristic of the Graptolite is not so significant 
as is assumed. 
