78 ZOOLOGY sect. 



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 nagellum and assume an amoeboid form. 



The nucleus (mt.) 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 

 Polyceca (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 Protcrospongia (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 flagella 

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

 very beautiful and complex shell. 



The body (Fig. 59, 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 groove (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 nagellum (fl. 1), 

 which is directed forwards and serves as the chief organ of propulsion ; a second 

 nagellum (fl. 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, and often of very 

 complex form, being produced into long and ornamental process, and marked 

 with stripes, dots, &c. Besides a nucleus and a contractile vacuole, the proto- 

 plasm contains chromatophores (1, chr.) coloured with chlorophyll or an allied 

 pigment of a yellow colour, called diatomin. Nutrition is holophytic or holozoic. 



The foregoing description applies to all the commoner genera. Prorocentrum 

 (3) is remarkable for the absence of the transverse groove, while Polykrikos (4) 

 has no fewer than eight transverse grooves and no shell. The latter genus 

 also has stinging-capsules or nematocysts (a, b) in the protoplasm, resembling 

 those of Zoophytes (see Sect. IV.), and has numerous nuclei of two sizes, 

 distinguished as meganuclei {nu. ), and micronuclei (nu'.). 



