PROTOZOA : GREGARINID^E. 5 I 



aspect of small worms. The integument or cuticle with which 

 the protoplasmic body is inclosed may be quite smooth or 

 striated, or it may be furnished with bristles or spines, or even 

 in some cases with cilia. Sometimes one end of the body is 

 furnished with uncinate processes, very similar in appearance 

 to the hooked "head" of the common tape-worm (T&nia 

 soliuni). 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 be effected, not, however, by pseudopodia. 

 Haeckel regards the Gregarince as Amoeba which have be- 

 come degenerate by parasitism ; but this opinion is rejected 

 by Van Beneden. 



In spite of their exceedingly simple structure, the following 

 very interesting reproductive phenomena have been observed 



Fig. 2. Gregarina of the earth-worm, a Adult Gregarina ; b The same encysted ; 

 r With the contents divided into pseudonavicellse ; d Free pseudcnavicellse ; 

 e Free amcebiform 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. In some species conjugation is invari- 

 able j in others it never occurs ; and it may take place either 

 by analogous or by opposite extremities. The Gregarina or 

 it may be two individuals which have come into contact and 

 adhered together assumes a globular form, becomes motion- 

 less, and develops round itself a structureless envelope or cyst, 

 when it is said to be "encysted" (fig. 2, b\ The central 

 nucleus then disappears, apparently by dissolution, whereupon 

 the granular contents of the cyst break up into a number of 

 little rounded masses, which gradually elongate and become 

 lanceolate, when they are termed "pseudonavicellae" (or 



