372 THE INFUSORIA 



this is usually their function in the Holotricha ; but it is quite 

 possible that in some cases they may be used primarily or entirely 

 as organs of defence. In Paramoecium (Fig. 46) they are evenly dis- 

 tributed in the cortex. In Prorodon they are confined to the anterior 

 end, and in the Amphileptina to the ventral side of the body. 



In Epistylis umbellaria among the PERITRICHA large oval nemato- 

 cysts 85 p in length have been described (Fig. 75). They bear a 

 thread which before the discharge is coiled up in a capsule in the 

 manner of the thread in the Coelenterate nematocyst. When the 

 nematocyst has exploded, the thread is eight to ten times the length 

 of the capsule. It is a remarkable fact that neither nematocysts 

 nor trichocysts have been hitherto found in any other genus of the 

 Peritricha. 



NUCLEI. In a large number of the genera included in the 

 Heterokaryota two distinct kinds of nucl'ei have been observed, 

 which differ from one another not only in size, but in form, structure, 

 and mode of division. They are called Mega-nuclei and Micro-nuclei 

 respectively. The micro-nuclei are frequently so small and so 

 difficult to distinguish from other particles in the protoplasm which 

 are stained by nuclear dyes, that their existence has been repeatedly 

 denied in species (Dendrocometes paradoxa, etc.) in which they are 

 undoubtedly present. The research of the last few years points to 

 the conclusion that all the Ciliata and Acinetaria are Heterokaryote. 



THE MEGANUCLEUS ( = MACRONUCLEUS). 



The shape of the meganucleus 1 varies greatly in the order. In 

 some genera, such as Paramoecium (Fig. 46), Trichonympha, and others, 

 it is either oval or spherical in shape during the whole of the 

 resting-stage. In Carchesium, V&rticella, and others it is much more 

 elongated, and assumes the form of a curved or bent sausage. In 

 Stentor (Fig. 44), Spirostomum, and others, it is moniliform during the 

 resting-stage, but contracts into an oval shape before normal fission 

 occurs. In several genera of Hypotricha and Holotricha the mega- 

 nucleus divides repeatedly after, or in some cases just before, fission, 

 and ultimately breaks up into numerous minute fragments scattered 

 through the medulla. This condition of fragmentation persists until 

 the animal is ready for another act of fission, when the fragments of 

 the meganucleus fuse together again into a single spherical or 

 oval body (Figs. 16, 17, and 18). 



A similar phenomenon of fragmentation of the nucleus has been 

 observed in the genus Opalinopsis (Fig. 19), but in this case unequal 

 fission may occur without the return of the nuclear fragments to 

 form a single compact mass. 



1 The prefix mega- (Greek /tryat = big) is preferable to macro- (Greek HULK pot = 

 long), and is less readily mistaken for micro-. 



