ANIMALCULES 495 



imbedded the green pear-shaped individuals, each of which is 

 provided with a pair of flagella, and is connected by threads of 

 protoplasm with its neighbours. The colony swims about by 

 means of the flagella, slowly revolving as it does so. We have 

 here an example of the most perfect type of symmetry known, 

 that of the sphere, a shape which is only possible when external 

 influences act on all parts alike, as can only be the case with a 

 revolving aquatic form. At times, however, the symmetry of 

 Volvox is disturbed by the specialization of certain cells for the 

 purpose of propagation. 



A small marine group of flagellates is represented by the 

 remarkable form Noctiluca (L. nox, night; lux, light), countless 

 myriads of which are sometimes found floating together in the 

 sea (fig. 301), and are one of the causes of the phosphorescence 

 common in the summer months. Each individual is of com- 

 paratively large size (-^ of an inch in diameter) and is shaped 

 like a peach, a very large fiagellum being attached at one end of 

 the groove. The mouth is situated at the base of this structure 

 and leads into a short gullet into which a second but smaller 

 flagellum projects. 



Group 2. — Amceba-like Protozoa (Rhizopoda) 



A description has already been given of the Proteus Animalcule 

 (Amoeba), which may be taken as a type of the group, all the 

 members of which are provided with those projections of the 

 body which are called pseudopods, though it is only in some 

 •cases that these are broad lobes, as in many species of Amoeba. 



There are a number of common fresh-water members of 

 this group (fig. 301), which essentially resemble Amoeba except 

 that they possess a shell of varied nature and shape. Sometimes 

 it consists of foreign particles cemented together, and at other 

 times it is entirely made up of material presumably of horny 

 nature formed by the activity of the protoplasm. 



Cases like those just mentioned lead on to a large and 

 important group found both in salt and fresh water, and char- 

 acterized by the presence of a shell that is often riddled with 

 minute pores or foramina, on which account the name of Fora- 

 minifera has been given (fig. 301). The shell may be either 

 tough and membranous, composed of foreign particles agglutinated 



