PROTOZOA. 51 



observations of Klebs and Biitschli a second flagellum 

 lies horizontally in this groove which has hitherto been 

 mistaken for a girdle of cilia. The body is covered with 

 a cellulose (Bergh) carapace formerly supposed to be 

 silicious or chitinous, and its shape is unique, having 

 three horns, two of which are in front and one behind. 

 Kent l has pointed out the isomorphic resemblance exist- 

 ing between the bodies of the Peridinidae and the larvae 

 of certain Echinodenns and Crustacea. The mechanical 

 conditions for a floating existence have probably been the 

 controlling cause of this peculiar shape of the body. 



PI. 52, fig. 2 (P. arcticum Ehr., dorsal view) has long 

 arms and the serrations seen in fig. i have here become 

 spines. According to Claparede and Lachmann these are 

 two of a large number of varieties of the species of Cera- 

 tium tripos. Although these two forms may occur in the 

 same locality, the Peridinium arcticum Ehr. is found most 

 abundantly in the colder, denser waters of the arctic seas, 

 where its broader and stouter arms probably assist in 

 preserving the equilibrium of the body. 



We come now to one of the members of the group of 

 Mastigophora which has long been known on account 

 of its remarkable property of brilliant phosphorescence. 

 The Noctiluca miliaris Sur. is cosmopolitan, and to it are 

 largely due the beautiful illuminations of the sea at night. 

 The young Noctiluca (PL 53, figs. 1-6) shows specializa- 

 tion in structure by the possession of a whip and a tenta- 

 cle-like organ near the mouth. The adult (PL 53, fig. 7) 

 has a transparent body in the form of a peach surrounded 

 by a distinct membrane. The protoplasm radiates from 

 the center of the body, and spreads itself in a layer over 

 the inner surface of the membrane (Kent). The mouth 

 is at the bottom of a depression where the flagellum 

 originates (which is not clearly seen in the drawing 2 ) and 



1 Manual of the Infusoria, I, 1880, p. 452. 



2 See Huxley, Quart. Journ. Micr. Sci., Ill, 1855, PL 5, fig. 3. 



