MASTIGOPHORA : DINOFLAGELLATA. 851 



except in Polykrikos, which has four, one behind the other. Each of these 

 four nuclei is said to have 3-6 small paranuclei apposed to it, as in some 

 Infusoria. The protoplasm is sometimes vacuolate ; and one or two large 

 vacuoles are common, whether contractile or not is uncertain 1 . Poly- 

 krikos has nematocysts similar in structure to those of Coelenterata, but 

 supposed by some authorities to be taken in with food. Some and prob- 

 ably all coloured Dinoflagellates are holophytic. Solid foreign bodies 

 have been observed in naked forms, and in two instances even their 

 actual ingestion 2 . 



Reproduction takes place by binary fission, longitudinal in Exuviaella 

 and Dinophysidae, oblique in most Peridinidae, but transverse in Hemi- 

 dinium and Polykrikos. The process rarely occurs except in Ceratium 

 and Polykrikos, while the animal is in a motile condition. It usually loses 

 its flagella, the protoplasm contracts from the cuticle and very generally 

 forms a cyst of a cellulose-like substance (?) within the cuticle, which is 

 detached by the swelling of the cyst before or after fission has been 

 accomplished. The cyst is in some instances gelatinous. In Peridinium 

 membranous semilunar cysts may be produced in which four, or in a 

 marine species eight, individuals are produced fissiparously. These cysts 

 appear to be attached temporarily by one horn 3 . The chromatophores 

 collect together and generally undergo resolution, the organism acquiring a 

 prevailing red tint. Fission is sometimes incomplete, resulting in double 

 individuals. It is uncertain whether conjugation ever takes place. No 

 such significance can be assigned to the chains of two or more individuals 

 observed in Ceratium and in one or two other instances ; they are prob- 

 ably formed, at least in the genus named, by fission (Bergh). 



Some Dinoflagellata have been observed encysted in a resting con- 

 dition. Examples .naked or protected by a fine cuticle are occasionally 

 met with, probably derived from such cysts. Growth, however, appears to 

 be accompanied by moults which may also be produced by unfavourable 

 conditions. Some marine species, e. g. Ceratium tripos, C. fusus, Proro- 

 centrum micans, are phosphorescent. Fossil species of Peridinium have 

 been recognised in rocks belonging to the cretaceous series in Germany. 



1 The two vacuoles usually flow together, and at the same time a fine canal is stated to be 

 visible leading to the surface, towards the bases of the flagella. The view that the vacuoles serve to 

 take in fluid nourishment, or are analogous to the sap- vacuoles of vegetable cells, is improbable. 



2 Bergh, e. g. observed solid masses of apparently ingested food in Gymnodinium gracile, 

 G. spirale, and Polykrikos atiricularia. An oesophageal tube is said to exist in Prorocentrum, and 

 by Entz in Gymnodinium pulvisctilus (Z. W. Z. xxxviii. p. 188). Kent observed his G. marinum 

 chasing monads, and swallowing them by a mouth near the base of the flagellum (Manual of In- 

 fusoria, i. p. 444), and Maupas observed an undetermined marine Peridinium devouring large 

 Infusoiia by means of a sucking tentacle (A. Z. Expt. ix. p. 365). 



3 Pyriform fixed cysts have been observed in Exiiviaella lima ; so too in Gymnodinium pulvis- 

 culus attached to the tail of an Appendicularia ; Pouchet, J. de 1'Anat. et Physiol. (Robin), xxi. 

 p. 6 1 et seqq. 



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