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 i-esembles 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'api)artient en propre a aucun 

 des Polypes reunis en masse, mais qui leur est commune a tons. 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 intervenir d'une maniere directe dans I'acte de la repro- 

 duction. ' Ann. dcs Sc. Nat. iv. p. 310. 1835. 



