CILIATA 



781 



like organs are developed, on which the animalcule creeps, as by 

 Ifgs, over solid surfaces; the external integument becomes more 

 consolidated on its upper surface, so as to become a kind of cara- 

 pace ; and a mouth is formed by the opening of a slit on one side, 

 in front of which is a single hair-like flagellum, which turns round 

 and round with great rapidity, so as to describe a soi-t of inverted 

 cone whereby a current is brought towards the mouth. This latter 

 form had been described by Professor Ehrenberg under the name of 

 Asjndisca. It is very much smaller than the larva, the difference 

 being, in fact, twice as great as that which exists between A and 



FIG. 507. -Metamorphoses of Trirlwda li/nceus : A, larva (Oxytricha) \ B,a 

 similar larva after swallowing the animalcule represented at M; C, a very 

 liii'Kf individual on the point of undergoing fission; D, another in which 

 the process has advanced further; E, one of the products of such fission; 

 F, the same body become spherical and motionless ; G, aspect of this 

 sphere fifteen days afterwards ; H, later condition of the same, showing the 

 formation of the cyst ; I, incipient separation between living substance 

 and exuvial matter ; K, partial discharge of the latter, with flattening of 

 the sphere ; L, more distinct formation of the confined animal ; M, its 

 escape from the cyst ; N, its appearance some days afterwards ; O, more 

 advanced stage of the same; P, Q, perfect Aspidisca, one as seen side- 

 ways, moving on its bristles, the other as seen from below (magnified 

 twice as much as the preceding figures). 



P, Q (fig. 597). since the last two figures are drawn under a magni- 

 fying power double that employed for the preceding. How the 

 Aspidisca-form in its turn gives origin to the Oxytricka-form 

 has not yet been made out. A similar 'encysting process' has 

 been observed to take place among several other forms of ciliated 

 Infusoria ; so that, considering the strong general resemblance 

 in kind and degree of organisation which prevails throughout the 

 group, it does not seem unlikely that it may occur at some stage of 

 the life of nearly all these animalcules. And it is not improbably 

 in the ' encysted ' condition that their dispersion chiefly takes place, 

 >ince they have been found to endure desiccation in this state, 

 although in their ordinary condition of activity they cannot be dried 



