100 LECTURE VIII. 



If one of these points be carefully turned out with a needle, it is found 

 to consist of a transparent sac, containing generally from four to six 

 of the gemmules. These are of a semi-oval form, with the margin of 

 their plain surface developed into tubercles supporting groups of 

 vibratile cilia. The body presents a simple granular structure ; the 

 genunule swims about actively by the vibration of its cilia, the motion 

 of which seems to be under its control. They generally swim with 

 the convex part forwards ; sometimes they simply rotate upon their 

 axis, or execute a series of summersets ; or, selecting a fixed point, 

 they whirl round it in rapid circles, carrying every loose particle after 

 them ; or they creep along the bottom of the watch glass upon one 

 end with a Avaddling gait : but at the expiration of forty-eight hours 

 they attach themselves to the surface of the glass, and the rudiments 

 of a cell may be observed. 



In the FLustra, the gemmules are developed between the cell and 

 the body of the polype, which yields to, and is destroyed by, them 

 as they are developed. These likewise escape, and, after a short term 

 of locomotive life, settle and subside, the outline of the cell being fii-st 

 formed, and the polype with its tentacula, muscles, and alimentary 

 canal being afterwards developed in a distinct small closed sac. 



There are a few genera of fresh-water polypes, as the Plumatella 

 and Cristatella, which have the ciliated tentacula in the form of cres- 

 centic or horse-shoe lobes. The Cristatella has been observed to 

 produce ova of a flattened discoid form, with their outer surface sin- 

 gularly beset with long bifurcated hooks like the infusorial Xatithidia. 

 The young Cristatella undergoes its metamorphosis from the ciliated 

 gemmule-state to the mature form of the polype in the ovum, from 

 which it escapes by splitting it into two parts. 



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, at least, as that of the monad, to one as high as 

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

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

 istic of the Bryozoa, as 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 characteristic 

 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 the compound MoUusca we find the ciliated tentacles re- 

 reduced to mere rudiments at the entrance of the alimentary canal ; 



