244 
ZOOPHYTA ASCIDIOIDA. 
traced in the Vesiculariadse, and in our common Flustrae and 
Escharinae, where round the margin of the crust, cells can at all 
seasons be observed in every stage of their evolution ; — one just 
jutting out, another half-formed, and others again nearly com- 
plete. They never begin their original in the body of the po- 
lype, but always from the parietes, or rather the connecting 
medium ; * nor indeed is the embryo distinguishable within 
until the cells have made considerable advances to maturity. 
Then the softer parts begin to assume a shape, and gradually 
to limb themselves after the similitude of their antecedent co- 
partners, when having reached their term and ready for a par- 
tial independency, they burst their outward cerements, always 
at a fixed point, prepared for their exit by the same Power 
which has moulded the whole. 
From this mode of increase there would seem to be no natu- 
ral limits set to the magnitude and duration of the polypidom, 
except what arise from accident or extrinsic causes. The ori- 
ginal polype and its immediate successors may grow old, lan- 
guish and die ; but the solid cells remain in their connection 
as a root and fixture, while the newer races, which have sprung 
up towards the outskirts, continue their work, — generation fol- 
lowing generation in rapid and ever-multiplying successions. 
The polypidom in this respect resembles a tree in its growth : 
the trunk and main branches have stood years and centuries, but 
the increase has been made by annual shoots and renewals, and 
the last know only vigour and juvenescence. And as the form 
of the tree depends on the fashion of its ramifications, so that of 
the polypidom on the mode of evolution of its cells, for every 
part of the axis is not equally organized to produce buds, nor 
the same parts in all. Hence if the primitive cell has only one 
point fitted for this gemmation, the polypidom will be builded 
up in a catenated chain ; if the cell has two points, two series 
* They, in this respect, are formed the same as the Asteroida, of which Milne- 
Edwards says : — “ On voit done qu’ ici la partie qui donne naissance aux bour- 
geons reproducteurs est precisement la partie qui n’appartient en propre a aucun 
des Polypes reunis en masse, mais qui leur est commune a tous. Le tissu gene- 
rateur entoure ces petits etres comme une sorte de gangue vivante et produit 
dans la profondeur de sa substance de nouveaux polypes sans qu’aucun de ceux 
deja existans paraissent inter venir d’une maniere directe dans 1’acte de la repro- 
duction.'’ Ann. des Sc. Nat. iv. p. 340. 1835. 
