80 ZOOLOGY SECT. 



the base of the single flagellum (Jl.). The collar is contractile, and, 

 although its precise functions are not yet certainly known, there is 

 evidence to show that its movements cause vortices in the water 

 which draw in small bodies towards the outside of the collar to which 

 they adhere. By degrees such bodies are drawn towards the base, 

 and each is received into a vacuole which moves back into the 

 interior of the protoplasm, another vacuole taking its place. The 

 animalcule may draw in both collar and flagellum and assume an 

 amoeboid form. 



The nucleus (nu.) is spherical, and there are one or two con- 

 tractile vacuoles (c. vac.), but no trace of mouth or gullet. Some 

 forms are naked (1), others (2) enclosed in a chitinoid shell or 

 lorica of cup-like form. A stalk (s.) is usually present in the 

 loricate and sometimes also in the naked forms. 

 * The genera mentioned in the preceding paragraph are all simple, 

 but in other cases colonies are produced by repeated fission. In 

 PolyoBca (3) the colony has a tree-like form, which may reach 

 a high degree of complexity by repeated branching. A totally 

 different mode of aggregation is found in Proterospongia (4), in 

 which the zooids are enclosed in a common gelatinous matrix of 

 irregular form. 



Reproduction. The " collared monads," as these organisms 

 are often called, multiply by longitudinal fission (2b). In some 

 cases multiple fission of encysted individuals has been observed 

 (2c), small simple flagellulse being produced which gradually 

 develop into the perfect form. 



The order is especially interesting from the fact that, with the 

 exception of Sponges, it is the only group in the animal kingdom 

 in which the collar occurs. 



ORDER 3. DINOFLAGELLATA. 



The leading features of this group are the arrangement of the two nagella 

 which they always possess, and the usual presence of a remarkable and often 

 very beautiful and complex shell. 



The body (Fig. 60, 1) is usually bilaterally asymmetrical, i.e. it may be 

 divided into right and left halves, which are not precisely similar. On the 

 ventral surface is a longitudinal groom (I. gr.), extending along the anterior half 

 only, and meeting a transverse groove (t. gr.), which is continued round the 

 body like a girdle. From the longitudinal groove springs a large flagellum 

 (fl.l), which is directed forwards and serves as the chief organ of propulsion ; 

 a second flagellum (Jl. 2) lies in the transverse groove, where its wave-like 

 movements formerly caused it to be mistaken for a ring of small cilia. 



The body is covered with a shell (2) formed of cellulose, sometimes silicified, 

 and often of very complex form, being produced into long and ornamental 

 processes, and marked with stripes, dots, &c. Besides a nucleus, a contractile 

 vacuole and often an eyespot, the protoplasm contains chromatophores 

 (1, chr.) coloured with a yellowish or brownish pigment. Nutrition is holo- 

 phytic or holozoic. 



The foregoing description applies to all the commoner genera. Proro- 

 centrum (3) is remarkable for the absence of the transverse groove, while 

 Polykrikos (4) has four to eight transverse grooves and no shell. The latter 



