H4 PRIMITIVE FORMS OF LIFE 



mouth (Spirostomum) , or be simply oblique (Bursaria). Finally, 

 when the Infusorian has become definitely ambulant, the cilia 

 of the dorsal side are atrophied as though through disuse, 

 and those of the ventral side take on special forms (hooks, 

 probes, cirri, paddles, etc.), adequate to the function they 

 fulfil. It is quite clear that we cannot assume for the Infusoria 

 anything comparable to volition, determining the use or disuse 

 of its organs, nor any sentiment of need. It is external 

 stimulation which incites to movement certain cilia rather 

 than others, and which determines the contraction or relaxing 

 of this or that part of the body. But use or disuse, although 

 determined by another cause, has had the same effect as in the 

 case of animals endowed with sensibility and volition. This 

 purely mechanical action is clearly exhibited in Sientor, a 

 large Infusorian provided with an adoral fringe analogous to 

 that of Spirostomum, and with a kind of posterior suction cup 

 that permits it to fix itself temporarily. When the animal 

 is free the cilia of the adoral fringe function like paddles 

 and propel it forward. When it is fixed behind by its sucker 

 they drag forward the body of the animal till it is stretched 

 and elongated into a kind of bell or trumpet, of which the adoral 

 fringe borders the base. This form becomes definitive in the 

 Vorticellids, which are almost permanently fixed. 



There is a very interesting difference between the free and 

 the fixed Infusorians as regards multiplication. All the ciliated 

 Infusorians multiply by division, and bipartition is the 

 normal type of this mode of multiplication. In the free 

 Infusorians it is produced transversely, and in the Vorticellids 

 longitudinally, so that in the former case the two new 

 Infusorians are placed one behind the other, and in the 

 second case side by side. However, in certain species, this 

 separation into new individuals is either retarded or does not 

 occur at all. In the first instance we find a chain of individuals 

 resembling the body of an annelid worm, with its division 

 into rings (Anoplophrya, Hoplitophrya, Opalinopsis) ; in the 

 second a kind of little bush (Zoothamnium, Carchesium, 

 Epistylis). We shall find the same forms allied with the same 

 conditions of life in the higher animals. 



Geometrically an egg floating in a homogeneous environ- 

 ment, such as water, ought to produce, after segmentation, a 

 hollow globe with walls formed of a single layer of blastomeres. 



