494 CHARACTERS OF INVERTEBRATE ANIMALS 



of at once becoming separated, as in Vorticella. All the members 

 of the colony are of exactly similar nature. 



Another large section of Infusorians, the Flagellata, is charac- 

 terized by the presence of those protoplasmic threads known as 

 flagella (p. 471), which, though allied to cilia, are capable of 

 executing much more complex movements, and are not present 

 in large numbers on the same cell. A common example is 

 Euglena (fig. 301), a minute green form with worm-like body, 

 found, sometimes very abundantly, in stagnant water. At the 

 front end there is one long flagellum, which acts as a swimming 

 organ, pulling the body after it through the* water. At its base 

 there is a minute mouth leading into an exceedingly short gullet, 

 near which is a pulsating vacuole, and a red pigment -spot. The 

 nucleus is central, and the green colouring matter (chlorophyll), 

 which is of the same nature as that found in ordinary plants, is 

 contained in specialized parts of the protoplasm. The body is 

 constantly altering its shape in a peculiar manner, but the presence 

 of a firm cuticle prevents the formation of pseudopods. 



Putrefying infusions contain vast numbers of very minute 

 flagellates, commonly known as monads (fig. 301). A well-known 

 form is the Springing Monad (Heteromita), so named from the 

 character of its movements. It is shaped like a bean, and pos- 

 sesses two long flagella, attached near the notched side. One 

 of these is extended forward during locomotion like the single 

 flagellum of Euglena, while the other is trailed behind. 



A very interesting section of the present group contains simple 

 and colonial forms, in which the individuals closely resemble the 

 collar-cells of sponges (see p. 484). Codonocladium (fig. 301) may 

 be taken as an example. The interest chiefly lies in the fact that 

 such forms perhaps give a hint as to the ancestry of the sponges. 



It has been mentioned that Euglena is like a green plant, 

 inasmuch as it contains chlorophyll, but the presence of a mouth 

 proves it to be an animal. There are, however, certain flagellates 

 which are coloured green by this pigment, and at the same time 

 are devoid of any mouth-opening. Indeed, many botanists claim 

 them as plants. Some are simple, others colonial, and of the 

 latter, one genus is particularly well known on account of its great 

 beauty as a microscopic object. This is Volvox, a form shaped 

 like a hollow sphere (fig. 301), and about the size of a very small 

 pin's head. At regular intervals in the wall of the sphere are 



