PROTOZOA I GREGAEINIDJE. 



41 



ture of a Gregarina, since it is almost exactly that of the un- 

 impregnated ovum (Jig. 1 a). An adult Gregarina, in fact, 

 may be said to be a single cell, consisting of an ill-defined 

 membranous envelope filled with a more or less granular sar- 

 code, which contains in its interior a vesicular nucleus, this 

 in turn enclosing a solid particle, or nucleolus. In some the 

 body exhibits an approach to a more complex structure by the 

 presence of internal septa; but it is doubtful whether this 

 appearance may not be due to the apposition and fusion of 

 two separate individuals. In others one end of the body is 

 furnished with uncinate processes, very similar in appearance 

 to the hooked 'head' of the common tape- worm (Tcenia 

 solium). Essentially, however, the structure of all appears to 

 be the same. No differentiated organs of any kind beyond 

 the nucleus and nucleolus exist, and both assimilation and 

 excretion must be performed simply by the general surface of 

 the body. The body is, nevertheless, contractile, and slow 

 movements can apparently be effected. 



In spite of their exceedingly simple structure, the following 

 very interesting reproductive phenomena have been observed, 



Fig. 1. Gregarina of the earth-worm, a. Adult Gregarina. b. The same encysted. 

 c. With the contents divided into pseudonavicellse. d. Free pseuclonavicellae. 

 e. Free aincebiform contents of the pseudonavicellae. (After Lieberkiihn.) 



sometimes in a single Gregarina without apparent cause, 

 sometimes as the result of the apposition and coalescence of 

 two individuals the exact nature of the process being in 

 either case obscure. The Gregarina or it may be two in- 

 lividuals which have come into contact and adhered together 

 ssumes a globular form, becomes motionless, and develops 

 >und itself a structureless envelope or cyst, when it is said 

 be 'encysted' (fig. 1 &). The central nucleus then dis- 

 )pears, apparently by dissolution, whereupon the granular 

 mtents of the cyst break up into a number of little rounded 

 which gradually elongate and become lanceolate, when 



