MASTIGOPHORA : DINOFLAGELLATA. 849 



one of its extremities, hence the anterior. One of them is extended 

 forwards, whilst the other is wound transversely round its base. The 

 Dinifera vary much in shape. The body is as a rule marked by two 

 furrows, a transverse or equatorial furrow, sometimes spiral to a greater or 

 less degree, and a ventral longitudinal furrow which unites the ventral 

 ends of the transverse furrow and extends backwards behind them, and 

 often to a variable degree in front of them. The two flagella are situated 

 at the spot where the furrows meet. The longitudinal flagellum extends 

 backwards along the corresponding furrow ; the transverse is lodged in the 

 transverse furrow, and encircles the body from left to right round the 

 dorsal aspect. It appears to be the principal agent in locomotion, and the 

 short wave-like undulations which pass along it from base to apex were 

 formerly interpreted as indicating a ciliation of the furrow ; hence the 

 name Cilioflagellata. The longitudinal flagellum is in some instances very 

 contractile ; the animal probably glides upon it as do some Flagellata. 

 There are three families of Dinifera. 



The Peridinidae are rarely naked (Gymnodinium, all species (?), Hemi- 

 dinitim). The cuticle is seldom structureless (G lenodinittin), but is usually 

 broken up into a number of plates arranged as a rule in a definite manner 

 on each side of the transverse furrow 1 . They vary however in number 

 size, &c., in different genera. An apical set crowns the anterior pole 

 of the body, i. e. that end which is usually directed forwards in locomotion. 

 They are generally prolonged into a conical tube open at its extremity. 

 An ant-apical set crowns similarly the opposite pole ; whilst two circles of 

 plates, a prae- and post-equatorial lie respectively in front of and behind 

 the equatorial furrow, which is usually placed near the centre of the body, 

 rarely anteriorly. When the furrow is absent the arrangement of the 

 plates marks the equatorial line. The furrows are lined by a delicate 

 cuticle, and in Ceratium the longitudinal usually expands into a sub- 

 triangular area. In the same genus the posterior pole is produced into a 

 horn, sometimes recurved ; and one or more horns may proceed from the 

 right and left post-equatorial plates. They contain prolongations of the 

 body. The aperture for the longitudinal flagellum is a slit. The second 

 family, the Dinophysidae, differs from the foregoing in several respects. 

 The body is elongate, and sometimes remarkably so (Amphisolenia). The 

 transverse furrow which is circular, is generally approximated to the ante- 

 rior pole, or almost terminal, and the longitudinal is very short 2 . The 

 aperture for the longitudinal flagellum is round and somewhat tubular. 

 The cuticle is wanting in Amphidinium alone ; it constitutes a bivalved 

 shell, and its valves are right and left. The margins of the furrows tend 



1 The structureless cuticle of Glenodinium is said to resolve itself into a number of plates 

 under certain conditions. 



a In the Peridinidans Oxytoxum and Ceratocorys a similar approximation is found, unless 

 Stein's view is right, and the furrow is posterior in these two genera. 



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