154 LECTURE VIII. 



Splitting it into two parts. Sometimes a second polype is developed 

 I by gemmation before this takes place ; and others bud out before the 

 1 colony becomes fixed : and in this free state it forms the Cristatella 

 mucedo of Cuvier. 



In thus tracing upwards the organisation of the animals which 

 present the common external character of a circle of radiated oral 

 tentacula, we have met with modifications of anatomical structure 

 which clearly indicate three classes, and conduct us from a grade of 

 organisation as low as that of the monad, to one as high as that of 

 the wheel-animalcule. We have already seen that certain forms of 

 the Rotifera, as the Stephanoceros, combine the external character- 

 istics of the Bryozoa, e. g. the cell and ciliated tentacula, with an 

 equally complicated type of internal organisation ; but no rotiferous 

 animal developes buds ; the Bryozoa still retain this common cha- 

 racteristic of the whole race of polypi. 

 ' The Bryozoa make a still closer approximation to the compound 

 Ascidians, which form the lowest step of the molluscous series : but 

 in these we find the ciliated tentacles reduced to mere rudiments at 

 the entrance of the alimentary canal ; whilst the pharynx, or first 

 division, is disproportionately enlarged, and, being highly vascular, 

 and beset with vibratile cilia, performs the chief part of the respira- 

 tory function. No compound Ascidian, moreover, quits the ovum, 

 as a gemmule swimming by means of cilia either generally diffused, 

 or aggregated on special lobes after the type of the rotifer ; and no 

 Bryozoon quits the ovum, in the guise of a cercarian, to swim abroad 

 by the alternate inflections of a caudal appendage. 



The metamorphoses which the Bryozoa undergo are essentially of 

 the same type as those of the lower Polypi : the embryo developed 

 from the ovum is an oval, discoid, or subdepressed body, with a 

 general or partial ciliated surface, by which it enjoys a brief locomo- 

 tive life after its liberation from the parent : the exceptions to this 

 rule being few, and confined to some fresh-water forms, e. g. Alcyo- 

 nella, which, thereby, depart further from the known course of 

 development in the Ascidia), and resemble rather the fresh-water 

 Hydrse, if M. Laurent be correct in representing the young of the 

 H. grisea as emerging, under the mature polype-form, from the 

 ovum.* 



The Anthozoa appear, without exception, to pass from the state of 

 the ovum to that of the ciliated locomotive "gemmule" as the larva 

 has been termed : most of them quit the parent in that guise : but in 

 the large Actinice they appear occasionally to undergo their trans- 



* CXXXI. pi. ii. figs. 11, 12, 13. 



