SUBKINGDOM PROTOZOA. 31 



the flagella are situated, and at the botton of which is situated 

 the mouth-opening. Noctiluca has the form of a cyst, pos- 

 sessing an external thin membrane- 

 like outer wall, to which branching 

 strands of protoplasm extend from 

 the central mass containing the 

 nucleus and lying slightly below the 

 depression which contains the fla- 

 gella. These are two in number, 

 one being short and whip-like, while 

 the other, usually known as the 

 " tentacle " (Fig. 15, t), is a highly 

 contractile, somewhat flattened, and, ^^^- i5. — IToctauca miliaris 

 relatively to the flagellum, thick pro- ^ ^ j^^j^X' """7" nucleus, 

 cess of the internal protoplasm. 



This structure is unrepresented in Leptodiscus, which other- 

 wise closely resembles Noctiluca. 



Noctiluca is of considerable physiological interest, since it is one of the 

 forms to which the phosphorescence of the ocean is due. The cause of the 

 light and its character are, however, as yet unknown. 



Reproduction in the Flagdlata.— The most frequent method 

 of reproduction in all the orders of the Flagellates is simple 

 division, either transverse or longitudinal. Encystment, fol- 

 lowed or not as the case may be by spore-formation, is also 

 common, and when accompanied by spore-formation may be 

 preceded by the conjugation and fusion of two individuals. 

 lu Oercomonas the spores are exceedingly abundant and small, 

 presenting the appearance of minute granules even under the 

 highest powers of the microscope, but in other forms, as 

 Ghlamydomonas, the spores are larger and much fewer in num- 

 ber, being only 4 or 8 in this particular case. An interesting 

 modification occurs in closely-related species (Fig. 16), some 

 individuals of which divide into a number of small spores 

 {microspores), while others undergo a more restricted division 

 and give rise to a few large spores (macrospores). The latter 

 develop directly into the adult forms, but the microspores 

 show a tendency to conjugate in pairs before undergoing 

 further development. This diiferentiation of two kinds of 

 spores is carried still farther in other forms where neither 



