68 Introduction to Animal ]\Iorphology. 



are the pigment spots in Ophryoglena, which may be- 

 photoscopic eyes, and consist of highly-refracting 

 granules on the convexity of a clear, watch-glass-like- 

 body. The latter structure exists in Bursaria without 

 pigment. Below the pharynx in Opercularia are two, 

 reniform, solid organs of unknown nature. A solid 

 body (nucleus) exists in all, sometimes movable, and 

 generally oval, band-like (Bursaria, Trachelius), or 

 string-like (Ophryoscolex), sometimes bent round the 

 pharynx (Vorticella). Within or beside the nucleus is 

 a rod-like nucleolus. 



Reproduction is by longitudinal, transverse, or 

 diagonal (Lagenophr}^s), fission, gemmation (Vorti- 

 cella), encystation, internal gemmation (Loxodes 

 bursaria, Cohn], or sexually by conjugation, often of 

 unequal individuals (Paramsecium). The nucleus 

 (ovary) splits into spheres, or ova ; the nucleolus, into 

 thread-like or rod-like spermatozoa. These become 

 interchanged in the two conjugating individuals, and 

 the ova develop. Stein describes having seen a brood- 

 canal for the extrusion of the germs. 



These animals abound in stagnant waters, infusions, 

 &c. Some are parasitic, and a few marine, some of 

 Avhich are phosphorescent. They may be divided pro- 

 visionally into four orders. 



I. Opalinasa — oval, parasitic, sluggish, and mouthless ; 

 with a ligulate nucleus, no contractile vesicle, but several 

 permanent saccular spaces, or a contractile canal in O. Plana- 

 riarum and O. uncinata. They may be holotrichal, or the 

 cilia on the under surface may become uncini (Alastor), or a 

 wavy undulating membrane may replace the cilia (Undulina). 

 A mouth-like depression, with a row of comb-like cilia, exists 

 in Plagiotoma. They increase by transverse fission, are found 

 in the intestines, &c., of Frogs, New^ts, Molluscs, &c., and 



