8 ZOOLOGY SECT. 



videnceto show that its movements cause vortices in the water which 

 raw in small bodies towards the outside of the collar to which they 

 dhere. By degrees such bodies are drawn towards the base, and 

 ach is received into a vacuole which moves back into the interior 

 f the protoplasm, another vacuole taking its place. The animalcule 

 lay draw in both collar and flagellum and assume an amoeboid form. 



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

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

 arms are naked (1), others (3) enclosed in a chitinoid shell or 

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

 jricate and sometimes also in the naked forms. 



The genera mentioned in the preceding paragraph are all simple, 

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

 ^olyara (o) the colony has a tree-like form, which may reach 



high degree of complexity by repeated branching. A totally 

 ifferent mode of aggregation is found in Proterospongia {J/.), in 

 rhich the zooids are enclosed in a common gelatinous matrix of 

 Tegular form. 



Reproduction. — The " collared monads," as these organisms 

 re often called, multiply by longitudinal iission {:'h). In some 

 ases multiple fission of encysted individuals has been observed 

 'c), small simple flagellulaa being produced which gradually 

 evelop into the perfect form. 



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

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

 1 which the collar occurs. 



Order 3. — Dinoflagellata. 



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

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

 ery beautiful and complex shell. 



The body (Fig. 59, 1) is usually bilaterally asymmetrical, i.e. it may be 

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

 antral surface is a longitudinal grooce (I. gr. ), extending along the anterior half 

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

 ke a girdle. From the longitudinal groove springs a large flagellum (/. 1), 

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

 igellum [fl. 2} lies in the transverse groove, where its wave-like movements 

 irmerly 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 

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

 ith stripes, dots, &o. Besides a nucleus and a contractile vacuole, the proto- 

 asm contains chromatophores {1, chr.) coloured with chlorophyll or an allied 

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



The foregoing description applies to all the commoner genera. Prorocentrum 

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

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

 30 has sfiiigiiig-capsutes or riematocysts [a, b) in the protoplasm, resembling 

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

 stinguished as meganudei (nu.), and micronucUi (nu'.). 



