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cuticle. This is exactly the form presented by the vesicles 

 (or rather cavities, for they have no proper membrane) just 

 before disappearing in the living Opalina, if seen in profile, 

 and seems to favour the idea of their opening externally. 

 When the cavity is reai^pearing, after collapse, it has a 

 globular form from the very first, increasing gradually at its 

 periphery, but maintaining its globular form, as seen in fig. 

 6, a, b, c, d, e, until the full dimensions are attained. On 

 some occasions during the collapse I have observed that this 

 takes place in a totally different way, the opposite walls 

 closing together, as in /, g, h. This mode of expansion and 

 contraction clearly agrees with the supposition that these 

 pulsating cavities are yielding points in the sarcode substance, 

 which are slowly distended by the accumulation of fluid; 

 the thin external wall ultimately bursting and allowing the 

 fluid to escape; and then the]adjacent parts unite again at once 

 as sarcodic matter is known to do, and a fresh accumulation 

 commences at the old place. The formation of elongated cavities 

 surrounding the nucleus, and having all the appearance of 

 the regular " vesicles," as well as the fusion of adjacent 

 cavities, and the continued distension of others after death, 

 by the excessive endosmose of liquid, all seem to favour the 

 same theory of the formation and mode of action of the 

 contractile cavities. 



The cavities vary much in number in the Opalinee I 

 studied ; very rarely they occurred on both sides of the body, 

 but were generally in a series of from five to ten on one side 

 only. Those in immediate proximity to one another appeared 

 to contract in succession, but there were sometimes two or 

 three points of departure, as it were. Frequently a gap 

 occurs in a series, which evidently ought to be filled u]3 by 

 the reapjjearance of a collapsed cavity ; but after long 

 watching it does not reappear. The time of contraction and 

 expansion of the same cavity varies, but the collapse occurs 

 a little less frequently in active Opalina? than twice a minute ; 

 thus one cavity or other is almost constantly contracting. 



So much Avith regard to the " cavities." The substance 

 in which these cavities are placed is almost homogeneous and 

 colourless, but in the living Opalina a layer of fine highly 

 refracting granules is seen to underlie this (g), and some of 

 the fine granules are scattered in the cortical substance. The 

 layer of granules marks out and bounds the great nucleus n, 

 which in the living creature is not very different in appear- 

 ance from the rest of the structure. A very slight addition 

 of acidulated water, however, brings out the nucleus very 

 distinctly, as in fig. 3. It is also rendered very obvious if 



