82 Mr. W. D. Lang—Revision of the 
they fuse in the middle line to form a calcareous, more 
or less perforate, intraterminal front wall. Such forms are 
Cribrimorphs. 
The evolutionary aim in the development of all families 
appears to be the disposal to the best advantage or least 
detriment to the organism of superfluous Calcium Car- 
bonate*. Doubtless, the ancestral Membranimorphs secreted 
the terminal spines under this stimulus. Continued pressure 
led to the exaggeration of the spines, and these, by bending 
over and fice with their opposing fellows, formed a median 
line of fusion which gave scope for a considerable deposition 
of Calcium Carbonate. 
Henceforward the disposal of Calcium Carbonate takes 
place in three main directions, which may be simultaneously 
pursued during the evolution of any one lineage. The 
intraterminal front wall tends to become more solidified by 
means of lateral fusions between the cost, and secondary 
Calcium Carbonate may be piled up on the median line of 
fusion. The distal apertural spines may thicken and fuse 
with each other or with secondary tissue outside them to 
form the distal shield of secondary aperture, while thickenings 
or processes of the apertural bar, often helped by fusions 
with avicularia or with the proximal pair of apertaral spines, 
form the proximal shield of a secondary aperture. And, 
thirdly, intereecial tissue may envelop the front wall 
peripherally or group itself around intercecial avicularia 
and, growing up, leave the intraterminal front wall sunk at 
a lower level. The secondary aperture may continue to 
grow in a tubular form, or may spread laterally and, by 
meeting and fusing with the outgrowths of neighbouring 
secondary apertures and intereecial tissue, may form a 
complete secondary front wall (lamina peristomica of 
Jullien) +; such forms are Steginomorphs, and occur inde- 
pendently in several genera, 
This general evolutionary history is corroborated in 
individual cases by the facts of development. Ontogeny 
is often used to elucidate the course of phylogeny, and the 
ontogenetic stages of individual cecia can sometimes be seen 
at the growing edge of the colony. But commoner and far 
more useful are the growth-stages of the colony itself. 
Astogeny {, like ontogeny, is frequently a useful guide to 
the phylogeny of the form considered, and in fossil Polyzoa 
* See Lang, 1916, Geol. Mag. dec. vi. vol. iii. pp. 74-77. 
+ Jullien, 1886, Bull. Soc. Zool. France, vol. xi. p. 609. 
{ Cumings, 1904, American Journ, Sci. vol. xvii. p. 50, 
