and Evolution wn Polyzoa. 75 
superfluous calcium carbonate of the organism’s after end in its new 
position. Even Gasteropod Molluscs to a certain extent can behave 
in this manner. The Arthropods wrestle chiefly with chitin and 
throw off its accumulation at each ecdysis; but such marine forms as 
add calcium salts to chitin tend to be cumbered more and more with 
their exoskeleton at the periodic moults in proportion to the amount of 
calcium present. Some Molluses, like the Rudiste, have succumbed 
to masses of calcium carbonate, but the most successful, apparently, 
have learnt how to circumvent its secretion and produce little or no 
shell. The point need not be laboured, but this aspect of the 
deposition of calcareous skeletal matter may help to explain much 
that is not otherwise clear in the structure and evolution of Polyzoa. 
The simplest Polyzoa—the less differentiated Cyclostomes—have 
tubular skeletons, and merely add to the length of their tube, 
occasionally inserting a diaphragm, and an entombing cap or ‘closure’ 
may be the moribund act of a senile individual. As the colonies 
become more massive it is required that the general zoarial surface 
should remain at the same level, and this could not be attained if 
each individual independently moved up its tube according to its 
capacity for calcium carbonate secretion. The forward movement would, 
therefore, be general and slow, and the deposit of calcium carbonate 
would be chiefly directed to the interzoccial spaces which generally 
would tend to widen as the tubular skeletons radiated from a common 
centre. This is the condition of the more complicated Cyclostomes 
and the Trepostomes and Cryptostomes in general. But, though 
the uneven projection of individuals from the surface of the colony 
is undesirable, skeletal projections that did not interfere with the 
currents and food supply would not only be harmless, but might even 
serve a protective purpose. Thus, when the pressure again became 
intolerable, acanthopores might arise. Again, if the insistence of 
calcium carbonate was severe, the whole of the ectocyst capable 
of, yet not actually, secreting would be in a state of unstable 
equilibrium, an external stimulus might start the deposition of 
calcium carbonate at any point, and an intrazoccial spine result. 
Cumings and Davenport do not suggest a utilitarian origin for these. 
‘* What possible functions these spines could have we do not venture 
to say.”” The renewal of the polypide may well have given the 
necessary stimulus for the formation of cystiphragms and been the 
excuse for depositing calcium carbonate even if there were no other 
need for it. Finally, it is unnecessary to look for any special utility 
in the massive secondary wall-thickening termed by the authors the 
cingulum, if the whole organism is (so to speak) aching to deposit it. 
Cumings and Davenport attribute the secondary thickness of the walls 
to senility of the individual, at least they say it accompanies senility. 
We claim, further, that it marks the senility of the lineage to which 
the individual belongs, and it is by turning to harmless or possibly 
useful account the same ever-present danger that other structures 
described in this paper are due. 
Such a conclusion has been impressed upon the writer by the study 
of Cretaceous Cheilostomes. The Cyclostomes and Trepostomes have 
been considered. The Cryptostomes divert their excess of calcium 
