204 



PROTOZOA. 



numbers at night as to make the whole surface light at the slight- 

 est agitation. The phosphorescence is apparently caused by oxi- 

 dation processes in the protroplasm, but it persists for some time 

 after deprivation of oxygen. The membrane covering the body is 

 interrupted by a pit at one point, the cytostome, near which is the 

 nucleus surrounded by an aggregation of protoplasm which sends 

 branching threads into the jelly of the body. At the entrance to 

 the cytostome is a flagellum, of no use in locomotion, and a band- 

 like tentacle consisting of an outgrowth of the body membrane 

 with a transversely banded muscular axis; it moves slowly with a 

 swinging motion. 



Noctiluca reproduces by simple fission and by formation of 

 swarm spores (fig. 142, B, (?, D). In the latter two individuals 

 lose tentacles, flagella, and cytostomes, and conjugate ; after 



mutual nuclear fertilization the 

 animals separate, while the proto- 

 plasm in each collects in a disc 

 which, by successive divisions, is 

 converted into numerous uni- 

 nucleate oval germs. These at first 

 project from the sphere, but later 

 separate and form small flagellate 

 spores whose later history is not 

 certainly known. 



Leptodisciis medusoidcs of Eu- 

 rope (fig. 143) has the appearance 

 of a medusa 1 to 1.5 mm. in 

 diameter. Its gelatinous disc is 

 covered by a membrane, and at the 

 FIG. uz.-Leptodiscus medusoides, sur- highest point of the concave surface 



face and optical section. /, nagellum; j s a mass O f protoplasm with a sin- 

 m, cytostome; n, nucleus; o, preoral 



tract; p, protoplasmic band. gi e nucleus. On one side of this a 



band goes to the mouth, on the other a canal bearing a fine 

 nagellum at its end. The animals swim well, like medusae, by 

 closing the umbrella, the motions of which are caused by delicate 

 muscles on the concave side. 



Class III. Ciliata. 



The Ciliata rival the Ehizopoda in numbers and variety of 

 form. They are so complicated in structure that they were long 

 held as multicellular, a view which was entirely abandoned only 

 in the last quarter-century. All have a form definite for the 



