850 THE ANIMAL KINGDOM. 



to become produced into lamellae. The lamellae of the transverse furrow 

 are directed forwards so as to form two cones one within the other ; the 

 anterior is the cephalic, the posterior the nuchal. The right margin of 

 the longitudinal furrow is but slightly produced ; its left greatly, and in 

 some instances into broad lamellae, which may be supported by three, 

 rarely more, rib-like thickenings. Cuticular lamellae may also be produced 

 along the junction of the valves of the shell posteriorly. The Polydinidae, 

 or third family of Dim/era, has but one genus, Polykrikos. It is barrel- 

 like in shape, compressed dorso-ventrally, and devoid of cuticle. The 

 body is encircled by eight somewhat spiral transverse furrows, each 

 lodging a transverse flagellum, and all connected by a longitudinal furrow. 

 The longitudinal flagellum is typically single, and situated a little in front 

 of the posterior end of the body. A second terminal and posterior 

 flagellum is sometimes present (Biitschli). 



The cuticle is brittle, colourless and hyaline, and at first structureless. 

 It generally developes delicate ridges, inclosing in Dinophysidae areolae, 

 with thicker ridges in the Peridinidae along the line of union of the plates. 

 Growth of the plates in the last-named family, and of the valves of Dino- 

 'pkysidae, is supposed to take place at the edges which are often trans- 

 versely striated 1 . In Dinophysidae each areola is perforated by a pore, 

 and pores are probably present also in Peridinidae. The protoplasm has 

 a clear layer of exoplasm in some naked forms, but probably not in other 

 instances. It may be reddish in hue when chromatophores are absent. 

 These last-named structures are as a rule numerous, small and placed 

 peripherally. A single and reticulate chromatophore liable to break up 

 occurs in Ceratium ; a single central one with out-runners in Pyrophacus, 

 &c., and two thin plate-like bodies in Exuviaella. Their colour varies 

 from brown to various tints of green, according as diatomin or chlorophyl 

 predominate in them. Unmixed chlorophyl occurs in the marine Proto- 

 peridinium viride. Colourless genera occur, e.g. Polykrikos, colourless 

 species in other genera, as well as colourless examples of normally coloured 

 species. Starch-granules, said to be lamellate in Ceratium, are present 

 even in colourless individuals. Oil drops, yellow or reddish, are found 

 especially in individuals about to pass into a resting condition, or actually 

 in that state. A drop of large size and tinted red with haematochrome, 

 the so-called eye-spot or stigma, is not infrequently present in freshwater 

 species beneath the longitudinal furrow. A black mass of pigment, with 

 an apposed transparent refractile body, supposed to be an eye, has been 

 observed in a marine species of Gymnodinium by Pouchet. The nucleus 

 is globular, ovate or band-like ; it has a reticulate structure, and is single 



1 The old plates, according to Joseph's account of Peridinium stygium, may remain over- 

 lying a new cuticle (Z. A. ii. p. 116). A lamination of the old cuticle has been noted in an old 

 Cerafittm tripos. 



