452 ORDER CILIO-FLAGELLATA. 



The above title is here conferred on the species of Peridinhim figured and briefly 

 described, without a name attached, by Dr. von Willemoes-Suhm in the ' Zeitschrift 

 fiir Wissenschafthche Zoologie,' Bd. xxi, p. 303, and Taf xxxi., 187 1. The even and 

 elegant symmetry of its form distinguishes it conspicuously from all other known 

 species of the genus. The character or even existence of the flagellum appears to 

 have escaped the notice of its discoverer. 



Genus VI. CERATIUM, Schrank.* 



Animalcules free-swimming, encuirassed, body divided by a central 

 ciliated furrow into two equal or subequal portions, a second shorter, non- 

 ciliated groove extending from the centre of the ventral aspect of the 

 equatorial furrow towards the anterior pole ; cuirass consisting of two equal 

 or subequal segments, separated by the equatorial groove, which may 

 or may not be subdivided into secondary plates or facets ; one or both of 

 these segments produced into more or less conspicuous horn-like prolonga- 

 tions ; flagellum single, inserted close to the oral aperture at or near the 

 junction of the equatorial and longitudinal furrows ; eye-like pigment-speck 

 rarely developed. Inhabiting salt and fresh water. 



The representatives of this genus, while formerly included in that of Peri- 

 dinunn, are conveniently separated and distinguished by the presence of the more 

 or less conspicuously developed horn-like extensions of the cuirass. The close 

 affinity of the two genera is at the same time abundantly demonstrated by such an 

 intermediate or annectant form as Cerathim divergens, which practically possesses 

 but a slightly more differentiated modification of the cuirass than obtains in the 

 more normal variety of Paidinium tabulatum. The larger number of species of this 

 genus are pelagic, many of them being noted for their phosphorescent properties. 

 An interesting feature connected with certain of these pelagic types is the marked 

 isomorphic resemblance presented by the cuirasses with their long, divergent, horn- 

 like prolongations, to other floating organisms with which they are associated. In this 

 manner the attenuately prolonged, triradiate cuirass of Ccratmm tripos (var. macroceros) 

 finds its countertype not only in the skeletal framework of the floating larva or 

 " Pluteus " of various Echinodermata, but in the larval or free-floating " zosea " 

 conditions of many of the highly organized Podophthalmous Crustacea, such as 

 Porcellana, in which a like production of the chief skeletal element, the carapace, 

 into three attenuate horn-like prolongations many times longer than the body, is 

 encountered. An even more astonishing mimetic resemblance subsists, however, 

 between the above-named species of Ceratium and the larval forms, the nauplii, or 

 so-called " Archezoa" (Dohrn) of the Cirripede Lcpas fascicidatus -z.^ figured by Dr. 

 R. von Willemoes-Suhm in the ' Philosophical Transactions,' pi. i., for the year 1876. 

 Here three long, serrated, spinous processes are developed in the same order and 

 maintain the same preponderating proportions with relation to the central body, the 

 superficial resemblance indicated being so complete that under a low magnifying 

 power the essentially distinct nature of the two organisms would scarcely be detected. 

 Finally, the four-horned Ceratium longicorne is obtained in company with a Rotifer 

 {Amirea longispiiia) characterized by the possession of great spinous processes that 

 attain a similar proportionate length, and decussate in the same order from the 

 surface of its carapace. The utility of these arm-like appendages, giving their 

 possessors greater buoyancy and marked fitness for a pelagic or floating existence, 

 is obvious ; their coincidence of plan in four such very divergent organic groups 

 being at the same time most remarkable. 



* 'Fauna Boica,' 1793. 



